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693b296b17
Add a limit on the number of simultaneously open xattr FIDs to prevent
host memory exhaustion attacks. Each xattr FID contains a buffer for the
xattr value, and without a limit, a malicious priviliged guest with
direct communication access to 9p server could create a huge number of
xattr FIDs until host memory is eventually exhausted.
Fix this by:
- add xattr_fid_limit to struct FsContext for the max. amount
- add xattr_fid_count to struct FsContext for the current amount
- init xattr_fid_limit with 1024
- init xattr_fid_count with 0
- add function xattr_fid_count_inc() to increment the count
- add function xattr_fid_count_decr() to decrement the count
- call xattr_fid_count_inc() in Txattrcreate handler
- call xattr_fid_count_inc() in Txattrwalk handler
- call xattr_fid_count_decr() when a xattr FID is freed
Additionally:
- reset the xattr FID counter in virtfs_reset()
When the limit is reached then xattr_fid_count_inc() returns -ENOSPC and
the request handler is aborted on its error path without turning the
FID into an xattr type and without allocating memory for the xattr.
The default value of 1024 was chosen, as (sane usage of) xattr requests
in the 9p protocol are usually very short-lived, and even machines with
128 cores with very high xattr activity should have plenty of head room
without ever hitting this limit.
Fixes: 10b468bdc5 ("virtio-9p: Implement TXATTRCREATE")
Fixes: CVE-2026-8348
Reported-by: Feifan Qian <bea1e@proton.me>
Link: https://lore.kernel.org/qemu-devel/eb3787869745d47234fb662600187bf773e1ef8a.1781361555.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>