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