diff options
author | Simon McVittie <smcv@collabora.com> | 2022-09-30 13:46:31 +0100 |
---|---|---|
committer | Simon McVittie <smcv@collabora.com> | 2022-10-05 10:47:05 +0100 |
commit | 51a5bbf9074855b0f4a353ed309938b196c13525 (patch) | |
tree | 3dcb68c72bf9ded853df45e4bfbacc84c2287522 | |
parent | 992c0da413d91c623431e809b60688700d64a1ec (diff) |
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 <smcv@collabora.com>
(cherry picked from commit 236f16e444e88a984cf12b09225e0f8efa6c5b44)
(cherry picked from commit 3fb065b0752db1e298e4ada52cf4adc414f5e946)
-rw-r--r-- | dbus/dbus-marshal-byteswap.c | 6 |
1 files changed, 1 insertions, 5 deletions
diff --git a/dbus/dbus-marshal-byteswap.c b/dbus/dbus-marshal-byteswap.c index 27695aaf..7104e9c6 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; |