Description

In the Linux kernel, the following vulnerability has been resolved:

nfs: fix UAF in direct writes

In production we have been hitting the following warning consistently

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 17 PID: 1800359 at lib/refcount.c:28 refcount_warn_saturate+0x9c/0xe0
Workqueue: nfsiod nfs_direct_write_schedule_work [nfs]
RIP: 0010:refcount_warn_saturate+0x9c/0xe0
PKRU: 55555554
Call Trace:

? __warn+0x9f/0x130
? refcount_warn_saturate+0x9c/0xe0
? report_bug+0xcc/0x150
? handle_bug+0x3d/0x70
? exc_invalid_op+0x16/0x40
? asm_exc_invalid_op+0x16/0x20
? refcount_warn_saturate+0x9c/0xe0
nfs_direct_write_schedule_work+0x237/0x250 [nfs]
process_one_work+0x12f/0x4a0
worker_thread+0x14e/0x3b0
? ZSTD_getCParams_internal+0x220/0x220
kthread+0xdc/0x120
? __btf_name_valid+0xa0/0xa0
ret_from_fork+0x1f/0x30

This is because we're completing the nfs_direct_request twice in a row.

The source of this is when we have our commit requests to submit, we
process them and send them off, and then in the completion path for the
commit requests we have

if (nfs_commit_end(cinfo.mds))
nfs_direct_write_complete(dreq);

However since we're submitting asynchronous requests we sometimes have
one that completes before we submit the next one, so we end up calling
complete on the nfs_direct_request twice.

The only other place we use nfs_generic_commit_list() is in
__nfs_commit_inode, which wraps this call in a

nfs_commit_begin...

Classification

CVE ID: CVE-2024-26958

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 5.08% (scored less or equal to compared to others)

EPSS Date: 2025-02-04 (when was this score calculated)

References

https://git.kernel.org/stable/c/4595d90b5d2ea5fa4d318d13f59055aa4bf3e7f5
https://git.kernel.org/stable/c/80d24b308b7ee7037fc90d8ac99f6f78df0a256f
https://git.kernel.org/stable/c/3abc2d160ed8213948b147295d77d44a22c88fa3
https://git.kernel.org/stable/c/e25447c35f8745337ea8bc0c9697fcac14df8605
https://git.kernel.org/stable/c/1daf52b5ffb24870fbeda20b4967526d8f9e12ab
https://git.kernel.org/stable/c/cf54f66e1dd78990ec6b32177bca7e6ea2144a95
https://git.kernel.org/stable/c/17f46b803d4f23c66cacce81db35fef3adb8f2af

Timeline