# CVE-2026-43420: ceph: fix i_nlink underrun during async unlink

Status: published. Published: 2026-05-08. Updated: 2026-11-06.

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

ceph: fix i_nlink underrun during async unlink

During async unlink, we drop the `i_nlink` counter before we receive
the completion (that will eventually update the `i_nlink`) because "we
assume that the unlink will succeed".  That is not a bad idea, but it
races against deletions by other clients (or against the completion of
our own unlink) and can lead to an underrun which emits a WARNING like
this one:

 WARNING: CPU: 85 PID: 25093 at fs/inode.c:407 drop_nlink+0x50/0x68
 Modules linked in:
 CPU: 85 UID: 3221252029 PID: 25093 Comm: php-cgi8.1 Not tainted 6.14.11-cm4all1-ampere #655
 Hardware name: Supermicro ARS-110M-NR/R12SPD-A, BIOS 1.1b 10/17/2023
 pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : drop_nlink+0x50/0x68
 lr : ceph_unlink+0x6c4/0x720
 sp : ffff80012173bc90
 x29: ffff80012173bc90 x28: ffff086d0a45aaf8 x27: ffff0871d0eb5680
 x26: ffff087f2a64a718 x25: 0000020000000180 x24: 0000000061c88647
 x23: 0000000000000002 x22: ffff07ff9236d800 x21: 0000000000001203
 x20: ffff07ff9237b000 x19: ffff088b8296afc0 x18: 00000000f3c93365
 x17: 0000000000070000 x16: ffff08faffcbdfe8 x15: ffff08faffcbdfec
 x14: 0000000000000000 x13: 45445f65645f3037 x12: 34385f6369706f74
 x11: 0000a2653104bb20 x10: ffffd85f26d73290 x9 : ffffd85f25664f94
 x8 : 00000000000000c0 x7 : 0000000000000000 x6 : 0000000000000002
 x5 : 0000000000000081 x4 : 0000000000000481 x3 : 0000000000000000
 x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff08727d3f91e8
 Call trace:
  drop_nlink+0x50/0x68 (P)
  vfs_unlink+0xb0/0x2e8
  do_unlinkat+0x204/0x288
  __arm64_sys_unlinkat+0x3c/0x80
  invoke_syscall.constprop.0+0x54/0xe8
  do_el0_svc+0xa4/0xc8
  el0_svc+0x18/0x58
  el0t_64_sync_handler+0x104/0x130
  el0t_64_sync+0x154/0x158

In ceph_unlink(), a call to ceph_mdsc_submit_request() submits the
CEPH_MDS_OP_UNLINK to the MDS, but does not wait for completion.

Meanwhile, between this call and the following drop_nlink() call, a
worker thread may process a CEPH_CAP_OP_IMPORT, CEPH_CAP_OP_GRANT or
just a CEPH_MSG_CLIENT_REPLY (the latter of which could be our own
completion).  These will lead to a set_nlink() call, updating the
`i_nlink` counter to the value received from the MDS.  If that new
`i_nlink` value happens to be zero, it is illegal to decrement it
further.  But that is exactly what ceph_unlink() will do then.

The WARNING can be reproduced this way:

1. Force async unlink; only the async code path is affected.  Having
   no real clue about Ceph internals, I was unable to find out why the
   MDS wouldn't give me the "Fxr" capabilities, so I patched
   get_caps_for_async_unlink() to always succeed.

   (Note that the WARNING dump above was found on an unpatched kernel,
   without this kludge - this is not a theoretical bug.)

2. Add a sleep call after ceph_mdsc_submit_request() so the unlink
   completion gets handled by a worker thread before drop_nlink() is
   called.  This guarantees that the `i_nlink` is already zero before
   drop_nlink() runs.

The solution is to skip the counter decrement when it is already zero,
but doing so without a lock is still racy (TOCTOU).  Since
ceph_fill_inode() and handle_cap_grant() both hold the
`ceph_inode_info.i_ceph_lock` spinlock while set_nlink() runs, this
seems like the proper lock to protect the `i_nlink` updates.

I found prior art in NFS and SMB (using `inode.i_lock`) and AFS (using
`afs_vnode.cb_lock`).  All three have the zero check as well.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < 9b31e88ac5623d15c8bc46f69dfe1d3b43a8f67c (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < 6d5fd8bb574bef039eb3b738e523870433a2aeb9 (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < fcc477a6e8856c8a42b3c9e171724d8d6dfadd06 (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < b3f5513141ecc6b277a8f7b7efe58a0cf9a5e859 (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < aedd29386b23f3e1e6818943e11abfff2953732f (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < 7db008e85a5d17b64bc5390b828bf457ae91a415 (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < 8975b85b0d45ca811ace6fac5907652f2310e5ac (affected, git); ≥ 2ccb45462aeaf0831397b90d31d3d50a7704fa1f; < ce0123cbb4a40a2f1bbb815f292b26e96088639f (affected, git). Source: CNA: Linux
- Linux / Linux: 5.7 (affected); ≥ 0; < 5.7 (unaffected, semver); ≥ 5.10.253; ≤ 5.10.* (unaffected, semver); ≥ 5.15.203; ≤ 5.15.* (unaffected, semver); ≥ 6.1.167; ≤ 6.1.* (unaffected, semver); ≥ 6.6.130; ≤ 6.6.* (unaffected, semver); ≥ 6.12.78; ≤ 6.12.* (unaffected, semver); ≥ 6.18.19; ≤ 6.18.* (unaffected, semver); ≥ 6.19.9; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.09% as of 2026-11-06; percentile 0.55%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/9b31e88ac5623d15c8bc46f69dfe1d3b43a8f67c (CNA: Linux)

- https://git.kernel.org/stable/c/6d5fd8bb574bef039eb3b738e523870433a2aeb9 (CNA: Linux)

- https://git.kernel.org/stable/c/fcc477a6e8856c8a42b3c9e171724d8d6dfadd06 (CNA: Linux)

- https://git.kernel.org/stable/c/b3f5513141ecc6b277a8f7b7efe58a0cf9a5e859 (CNA: Linux)

- https://git.kernel.org/stable/c/aedd29386b23f3e1e6818943e11abfff2953732f (CNA: Linux)

- https://git.kernel.org/stable/c/7db008e85a5d17b64bc5390b828bf457ae91a415 (CNA: Linux)

- https://git.kernel.org/stable/c/8975b85b0d45ca811ace6fac5907652f2310e5ac (CNA: Linux)

- https://git.kernel.org/stable/c/ce0123cbb4a40a2f1bbb815f292b26e96088639f (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-43420; retrieved 2026-11-06; source updated 2026-11-06

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-43420; retrieved 2026-11-06; source updated Not reported

Revision: ba5e874174e62a0f6e9149f61f8cce5d00924a8bc5790e32904714d1663584d5

Canonical: https://y2.dev/cve/CVE-2026-43420

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