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@ -1,330 +0,0 @@
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From 8451fa0ae30397b83705a193aa0d3f7752486dda Mon Sep 17 00:00:00 2001
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From: Alexander Larsson <alexl@redhat.com>
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Date: Mon, 3 Jun 2024 12:22:30 +0200
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Subject: [PATCH 1/4] Don't follow symlinks when mounting persisted directories
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These directories are in a location under application control, so we
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can't trust them to not be a symlink outside of the files accessibe to
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the application.
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Continue to treat --persist=/foo as --persist=foo for backwards compat,
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since this is how it (accidentally) worked before, but print a warning.
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Don't allow ".." elements in persist paths: these would not be useful
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anyway, and are unlikely to be in use, however they could potentially
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be used to confuse the persist path handling.
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This partially addresses CVE-2024-42472. If only one instance of the
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malicious or compromised app is run at a time, the vulnerability
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is avoided. If two instances can run concurrently, there is a
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time-of-check/time-of-use issue remaining, which can only be resolved
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with changes to bubblewrap; this will be resolved in a separate commit,
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because the bubblewrap dependency might be more difficult to provide in
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LTS distributions.
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Helps: CVE-2024-42472, GHSA-7hgv-f2j8-xw87
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[smcv: Make whitespace consistent]
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[smcv: Use g_warning() if unable to create --persist paths]
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[smcv: Use stat() to detect symlinks and warn about them]
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[smcv: Use glnx_steal_fd() for portability to older GLib]
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Co-authored-by: Simon McVittie <smcv@collabora.com>
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Signed-off-by: Simon McVittie <smcv@collabora.com>
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---
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common/flatpak-context.c | 108 +++++++++++++++++++++++++++++++++++++--
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1 file changed, 105 insertions(+), 3 deletions(-)
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diff --git a/common/flatpak-context.c b/common/flatpak-context.c
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index 53b79807..8c784acf 100644
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--- a/common/flatpak-context.c
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+++ b/common/flatpak-context.c
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@@ -2686,6 +2686,90 @@ flatpak_context_get_exports_full (FlatpakContext *context,
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return g_steal_pointer (&exports);
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}
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+/* This creates zero or more directories unders base_fd+basedir, each
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+ * being guaranteed to either exist and be a directory (no symlinks)
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+ * or be created as a directory. The last directory is opened
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+ * and the fd is returned.
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+ */
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+static gboolean
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+mkdir_p_open_nofollow_at (int base_fd,
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+ const char *basedir,
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+ int mode,
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+ const char *subdir,
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+ int *out_fd,
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+ GError **error)
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+{
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+ glnx_autofd int parent_fd = -1;
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+
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+ if (g_path_is_absolute (subdir))
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+ {
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+ const char *skipped_prefix = subdir;
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+
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+ while (*skipped_prefix == '/')
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+ skipped_prefix++;
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+
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+ g_warning ("--persist=\"%s\" is deprecated, treating it as --persist=\"%s\"", subdir, skipped_prefix);
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+ subdir = skipped_prefix;
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+ }
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+
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+ g_autofree char *subdir_dirname = g_path_get_dirname (subdir);
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+
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+ if (strcmp (subdir_dirname, ".") == 0)
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+ {
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+ /* It is ok to open basedir with follow=true */
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+ if (!glnx_opendirat (base_fd, basedir, TRUE, &parent_fd, error))
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+ return FALSE;
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+ }
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+ else if (strcmp (subdir_dirname, "..") == 0)
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+ {
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+ return glnx_throw (error, "'..' not supported in --persist paths");
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+ }
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+ else
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+ {
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+ if (!mkdir_p_open_nofollow_at (base_fd, basedir, mode,
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+ subdir_dirname, &parent_fd, error))
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+ return FALSE;
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+ }
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+
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+ g_autofree char *subdir_basename = g_path_get_basename (subdir);
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+
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+ if (strcmp (subdir_basename, ".") == 0)
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+ {
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+ *out_fd = glnx_steal_fd (&parent_fd);
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+ return TRUE;
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+ }
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+ else if (strcmp (subdir_basename, "..") == 0)
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+ {
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+ return glnx_throw (error, "'..' not supported in --persist paths");
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+ }
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+
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+ if (!glnx_shutil_mkdir_p_at (parent_fd, subdir_basename, mode, NULL, error))
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+ return FALSE;
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+
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+ int fd = openat (parent_fd, subdir_basename, O_PATH | O_NONBLOCK | O_DIRECTORY | O_CLOEXEC | O_NOCTTY | O_NOFOLLOW);
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+ if (fd == -1)
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+ {
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+ int saved_errno = errno;
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+ struct stat stat_buf;
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+
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+ /* If it's a symbolic link, that could be a user trying to offload
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+ * large data to another filesystem, but it could equally well be
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+ * a malicious or compromised app trying to exploit GHSA-7hgv-f2j8-xw87.
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+ * Produce a clearer error message in this case.
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+ * Unfortunately the errno we get in this case is ENOTDIR, so we have
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+ * to ask again to find out whether it's really a symlink. */
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+ if (saved_errno == ENOTDIR &&
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+ fstatat (parent_fd, subdir_basename, &stat_buf, AT_SYMLINK_NOFOLLOW) == 0 &&
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+ S_ISLNK (stat_buf.st_mode))
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+ return glnx_throw (error, "Symbolic link \"%s\" not allowed to avoid sandbox escape", subdir_basename);
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+
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+ return glnx_throw_errno_prefix (error, "openat(%s)", subdir_basename);
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+ }
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+
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+ *out_fd = fd;
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+ return TRUE;
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+}
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+
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void
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flatpak_context_append_bwrap_filesystem (FlatpakContext *context,
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FlatpakBwrap *bwrap,
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@@ -2709,12 +2793,30 @@ flatpak_context_append_bwrap_filesystem (FlatpakContext *context,
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while (g_hash_table_iter_next (&iter, &key, NULL))
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{
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const char *persist = key;
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- g_autofree char *src = g_build_filename (g_get_home_dir (), ".var/app", app_id, persist, NULL);
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+ g_autofree char *appdir = g_build_filename (g_get_home_dir (), ".var/app", app_id, NULL);
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g_autofree char *dest = g_build_filename (g_get_home_dir (), persist, NULL);
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+ g_autoptr(GError) local_error = NULL;
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+
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+ if (g_mkdir_with_parents (appdir, 0755) != 0)
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+ {
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+ g_warning ("Unable to create directory %s", appdir);
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+ continue;
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+ }
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+
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+ /* Don't follow symlinks from the persist directory, as it is under user control */
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+ glnx_autofd int src_fd = -1;
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+ if (!mkdir_p_open_nofollow_at (AT_FDCWD, appdir, 0755,
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+ persist, &src_fd,
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+ &local_error))
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+ {
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+ g_warning ("Failed to create persist path %s: %s", persist, local_error->message);
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+ continue;
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+ }
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- g_mkdir_with_parents (src, 0755);
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+ g_autofree char *src_via_proc = g_strdup_printf ("/proc/self/fd/%d", src_fd);
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- flatpak_bwrap_add_bind_arg (bwrap, "--bind", src, dest);
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+ flatpak_bwrap_add_fd (bwrap, glnx_steal_fd (&src_fd));
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+ flatpak_bwrap_add_bind_arg (bwrap, "--bind", src_via_proc, dest);
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}
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}
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--
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2.46.0
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From 5462c9b1e1a34b1104c8a0843a10382e90c9bb6b Mon Sep 17 00:00:00 2001
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From: Alexander Larsson <alexl@redhat.com>
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Date: Mon, 3 Jun 2024 12:59:05 +0200
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Subject: [PATCH 2/4] Add test coverage for --persist
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This adds three "positive" tests: the common case --persist=.persist, the
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deprecated spelling --persist=/.persist, and the less common special case
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--persist=. as used by Steam.
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It also adds "negative" tests for CVE-2024-42472: if the --persist
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directory is a symbolic link or contains path segment "..", we want that
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to be rejected.
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Reproduces: CVE-2024-42472, GHSA-7hgv-f2j8-xw87
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[smcv: Add "positive" tests]
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[smcv: Exercise --persist=..]
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[smcv: Assert that --persist with a symlink produces expected message]
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Co-authored-by: Simon McVittie <smcv@collabora.com>
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Signed-off-by: Simon McVittie <smcv@collabora.com>
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---
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tests/test-run.sh | 41 ++++++++++++++++++++++++++++++++++++++++-
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1 file changed, 40 insertions(+), 1 deletion(-)
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diff --git a/tests/test-run.sh b/tests/test-run.sh
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index dd371df3..bca0845d 100644
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--- a/tests/test-run.sh
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+++ b/tests/test-run.sh
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@@ -24,7 +24,7 @@ set -euo pipefail
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skip_without_bwrap
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skip_revokefs_without_fuse
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-echo "1..20"
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+echo "1..21"
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# Use stable rather than master as the branch so we can test that the run
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# command automatically finds the branch correctly
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@@ -512,3 +512,42 @@ ${FLATPAK} ${U} info -m org.test.App > out
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assert_file_has_content out "^sdk=org\.test\.Sdk/$(flatpak --default-arch)/stable$"
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ok "--sdk option"
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+
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+rm -fr "$HOME/.var/app/org.test.Hello"
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+mkdir -p "$HOME/.var/app/org.test.Hello"
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+run --command=sh --persist=.persist org.test.Hello -c 'echo can-persist > .persist/rc'
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+sed -e 's,^,#--persist=.persist# ,g' < "$HOME/.var/app/org.test.Hello/.persist/rc" >&2
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+assert_file_has_content "$HOME/.var/app/org.test.Hello/.persist/rc" "can-persist"
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+
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+ok "--persist=.persist persists a directory"
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+
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+rm -fr "$HOME/.var/app/org.test.Hello"
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+mkdir -p "$HOME/.var/app/org.test.Hello"
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+# G_DEBUG= to avoid the deprecation warning being fatal
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+G_DEBUG= run --command=sh --persist=/.persist org.test.Hello -c 'echo can-persist > .persist/rc'
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+sed -e 's,^,#--persist=/.persist# ,g' < "$HOME/.var/app/org.test.Hello/.persist/rc" >&2
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+assert_file_has_content "$HOME/.var/app/org.test.Hello/.persist/rc" "can-persist"
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+
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+ok "--persist=/.persist is a deprecated form of --persist=.persist"
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+
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+rm -fr "$HOME/.var/app/org.test.Hello"
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+mkdir -p "$HOME/.var/app/org.test.Hello"
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+run --command=sh --persist=. org.test.Hello -c 'echo can-persist > .persistrc'
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+sed -e 's,^,#--persist=.# ,g' < "$HOME/.var/app/org.test.Hello/.persistrc" >&2
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+assert_file_has_content "$HOME/.var/app/org.test.Hello/.persistrc" "can-persist"
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+
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+ok "--persist=. persists all files"
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+
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+mkdir "${TEST_DATA_DIR}/inaccessible"
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+echo FOO > ${TEST_DATA_DIR}/inaccessible/secret-file
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+rm -fr "$HOME/.var/app/org.test.Hello"
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+mkdir -p "$HOME/.var/app/org.test.Hello"
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+ln -fns "${TEST_DATA_DIR}/inaccessible" "$HOME/.var/app/org.test.Hello/persist"
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+# G_DEBUG= to avoid the warnings being fatal when we reject a --persist option.
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+# LC_ALL=C so we get the expected non-localized string.
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+LC_ALL=C G_DEBUG= run --command=ls --persist=persist --persist=relative/../escape org.test.Hello -la ~/persist &> hello_out || true
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+sed -e 's,^,#--persist=symlink# ,g' < hello_out >&2
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+assert_file_has_content hello_out "not allowed to avoid sandbox escape"
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+assert_not_file_has_content hello_out "secret-file"
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+
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+ok "--persist doesn't allow sandbox escape via a symlink (CVE-2024-42472)"
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--
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2.46.0
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From 04d8ad3009cd8a4350fba6cf7cc6c7819ccdfd34 Mon Sep 17 00:00:00 2001
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From: Simon McVittie <smcv@collabora.com>
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Date: Mon, 12 Aug 2024 19:48:18 +0100
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Subject: [PATCH 3/4] build: Require a version of bubblewrap with the --bind-fd
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option
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We need this for the --bind-fd option, which will close a race
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condition in our solution to CVE-2024-42472.
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For this stable branch, check the --help output for a --bind-fd option
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instead of requiring a specific version number, to accommodate possible
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backports in LTS distributions.
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Signed-off-by: Simon McVittie <smcv@collabora.com>
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---
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configure.ac | 3 +++
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1 file changed, 3 insertions(+)
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diff --git a/configure.ac b/configure.ac
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index 0a44e11a..0c8e2d0e 100644
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--- a/configure.ac
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+++ b/configure.ac
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@@ -175,6 +175,9 @@ if test "x$BWRAP" != xfalse; then
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BWRAP_VERSION=`$BWRAP --version | sed 's,.*\ \([0-9]*\.[0-9]*\.[0-9]*\)$,\1,'`
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AX_COMPARE_VERSION([$SYSTEM_BWRAP_REQS],[gt],[$BWRAP_VERSION],
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[AC_MSG_ERROR([You need at least version $SYSTEM_BWRAP_REQS of bubblewrap to use the system installed version])])
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+ AS_IF([$BWRAP --help | grep '@<:@-@:>@-bind-fd' >/dev/null],
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+ [:],
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+ [AC_MSG_ERROR([$BWRAP does not list required option --bind-fd in its --help])])
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AM_CONDITIONAL([WITH_SYSTEM_BWRAP], [true])
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else
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AC_CHECK_LIB(cap, cap_from_text, CAP_LIB=-lcap)
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--
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2.46.0
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From 2772f19e50c0e809dde8cf3c105d90ee8baf4fa8 Mon Sep 17 00:00:00 2001
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From: Simon McVittie <smcv@collabora.com>
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Date: Wed, 14 Aug 2024 13:44:30 +0100
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Subject: [PATCH 4/4] persist directories: Pass using new bwrap --bind-fd
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option
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Instead of passing a /proc/self/fd bind mount we use --bind-fd, which
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has two advantages:
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* bwrap closes the fd when used, so it doesn't leak into the started app
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* bwrap ensures that what was mounted was the passed in fd (same dev/ino),
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as there is a small (required) gap between symlink resolve and mount
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where the target path could be replaced.
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Please note that this change requires an updated version of bubblewrap.
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Resolves: CVE-2024-42472, GHSA-7hgv-f2j8-xw87
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[smcv: Make whitespace consistent]
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Co-authored-by: Simon McVittie <smcv@collabora.com>
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Signed-off-by: Simon McVittie <smcv@collabora.com>
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---
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|
common/flatpak-context.c | 4 ++--
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1 file changed, 2 insertions(+), 2 deletions(-)
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diff --git a/common/flatpak-context.c b/common/flatpak-context.c
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index 8c784acf..baa62728 100644
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--- a/common/flatpak-context.c
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+++ b/common/flatpak-context.c
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@@ -2813,10 +2813,10 @@ flatpak_context_append_bwrap_filesystem (FlatpakContext *context,
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continue;
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}
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- g_autofree char *src_via_proc = g_strdup_printf ("/proc/self/fd/%d", src_fd);
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+ g_autofree char *src_via_proc = g_strdup_printf ("%d", src_fd);
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flatpak_bwrap_add_fd (bwrap, glnx_steal_fd (&src_fd));
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- flatpak_bwrap_add_bind_arg (bwrap, "--bind", src_via_proc, dest);
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+ flatpak_bwrap_add_bind_arg (bwrap, "--bind-fd", src_via_proc, dest);
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}
|
|
|
|
|
}
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|
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|
|
|
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|
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--
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|
2.46.0
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