CVE-2026-43420

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 wouldnt 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.

Weakness

The product checks the state of a resource before using that resource, but the resource’s state can change between the check and the use in a way that invalidates the results of the check.

Affected Software

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/27.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/6d5fd8bb574bef039eb3b738e523870433a2aeb9
• https://git.kernel.org/stable/c/7db008e85a5d17b64bc5390b828bf457ae91a415
• https://git.kernel.org/stable/c/8975b85b0d45ca811ace6fac5907652f2310e5ac
• https://git.kernel.org/stable/c/9b31e88ac5623d15c8bc46f69dfe1d3b43a8f67c
• https://git.kernel.org/stable/c/aedd29386b23f3e1e6818943e11abfff2953732f
• https://git.kernel.org/stable/c/b3f5513141ecc6b277a8f7b7efe58a0cf9a5e859
• https://git.kernel.org/stable/c/ce0123cbb4a40a2f1bbb815f292b26e96088639f
• https://git.kernel.org/stable/c/fcc477a6e8856c8a42b3c9e171724d8d6dfadd06