74 lines
3.3 KiB
Diff
74 lines
3.3 KiB
Diff
From 51a5bbf9074855b0f4a353ed309938b196c13525 Mon Sep 17 00:00:00 2001
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From: Simon McVittie <smcv@collabora.com>
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Date: Fri, 30 Sep 2022 13:46:31 +0100
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Subject: [PATCH] dbus-marshal-byteswap: Byte-swap Unix fd indexes if needed
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When a D-Bus message includes attached file descriptors, the body of the
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message contains unsigned 32-bit indexes pointing into an out-of-band
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array of file descriptors. Some D-Bus APIs like GLib's GDBus refer to
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these indexes as "handles" for the associated fds (not to be confused
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with a Windows HANDLE, which is a kernel object).
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The assertion message removed by this commit is arguably correct up to
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a point: fd-passing is only reasonable on a local machine, and no known
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operating system allows processes of differing endianness even on a
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multi-endian ARM or PowerPC CPU, so it makes little sense for the sender
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to specify a byte-order that differs from the byte-order of the recipient.
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However, this doesn't account for the fact that a malicious sender
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doesn't have to restrict itself to only doing things that make sense.
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On a system with untrusted local users, a message sender could crash
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the system dbus-daemon (a denial of service) by sending a message in
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the opposite endianness that contains handles to file descriptors.
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Before this commit, if assertions are enabled, attempting to byteswap
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a fd index would cleanly crash the message recipient with an assertion
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failure. If assertions are disabled, attempting to byteswap a fd index
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would silently do nothing without advancing the pointer p, causing the
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message's type and the pointer into its contents to go out of sync, which
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can result in a subsequent crash (the crash demonstrated by fuzzing was
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a use-after-free, but other failure modes might be possible).
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In principle we could resolve this by rejecting wrong-endianness messages
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from a local sender, but it's actually simpler and less code to treat
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wrong-endianness messages as valid and byteswap them.
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Thanks: Evgeny Vereshchagin
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Fixes: ba7daa60 "unix-fd: add basic marshalling code for unix fds"
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Resolves: https://gitlab.freedesktop.org/dbus/dbus/-/issues/417
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Resolves: CVE-2022-42012
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Signed-off-by: Simon McVittie <smcv@collabora.com>
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(cherry picked from commit 236f16e444e88a984cf12b09225e0f8efa6c5b44)
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(cherry picked from commit 3fb065b0752db1e298e4ada52cf4adc414f5e946)
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---
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dbus/dbus-marshal-byteswap.c | 6 +-----
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1 file changed, 1 insertion(+), 5 deletions(-)
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diff --git a/dbus/dbus-marshal-byteswap.c b/dbus/dbus-marshal-byteswap.c
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index 27695aafb..7104e9c63 100644
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--- a/dbus/dbus-marshal-byteswap.c
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+++ b/dbus/dbus-marshal-byteswap.c
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@@ -61,6 +61,7 @@ byteswap_body_helper (DBusTypeReader *reader,
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case DBUS_TYPE_BOOLEAN:
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case DBUS_TYPE_INT32:
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case DBUS_TYPE_UINT32:
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+ case DBUS_TYPE_UNIX_FD:
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{
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p = _DBUS_ALIGN_ADDRESS (p, 4);
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*((dbus_uint32_t*)p) = DBUS_UINT32_SWAP_LE_BE (*((dbus_uint32_t*)p));
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@@ -188,11 +189,6 @@ byteswap_body_helper (DBusTypeReader *reader,
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}
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break;
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- case DBUS_TYPE_UNIX_FD:
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- /* fds can only be passed on a local machine, so byte order must always match */
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- _dbus_assert_not_reached("attempted to byteswap unix fds which makes no sense");
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- break;
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-
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default:
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_dbus_assert_not_reached ("invalid typecode in supposedly-validated signature");
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break;
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--
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GitLab
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