CVE-2026-46052

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

ceph: only d_add() negative dentries when they are unhashed

Ceph can call d_add(dentry, NULL) on a negative dentry that is already

present in the primary dcache hash.

In the current VFS that is not safe. d_add() goes through __d_add()

to __d_rehash(), which unconditionally reinserts dentry->d_hash into

the hlist_bl bucket. If the dentry is already hashed, reinserting the

same node can corrupt the bucket, including creating a self-loop.

Once that happens, __d_lookup() can spin forever in the hlist_bl walk,

typically looping only on the d_name.hash mismatch check and

eventually triggering RCU stall reports like this one:

rcu: INFO: rcu_sched self-detected stall on CPU

rcu: 87-....: (2100 ticks this GP) idle=3a4c/1/0x4000000000000000 softirq=25003319/25003319 fqs=829

rcu: (t=2101 jiffies g=79058445 q=698988 ncpus=192)

CPU: 87 UID: 2952868916 PID: 3933303 Comm: php-cgi8.3 Not tainted 6.18.17-i1-amd #950 NONE

Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.6 09/22/2023

RIP: 0010:__d_lookup+0x46/0xb0

Code: c1 e8 07 48 8d 04 c2 48 8b 00 49 89 fc 49 89 f5 48 89 c3 48 83 e3 fe 48 83 f8 01 77 0f eb 2d 0f 1f 44 00 00 48 8b 1b 48 85 db <74> 20 39 6b 18 75 f3 48 8d 7b 78 e8 ba 85 d0 00 4c 39 63 10 74 1f

RSP: 0018:ff745a70c8253898 EFLAGS: 00000282

RAX: ff26e470054cb208 RBX: ff26e470054cb208 RCX: 000000006e958966

RDX: ff26e48267340000 RSI: ff745a70c82539b0 RDI: ff26e458f74655c0

RBP: 000000006e958966 R08: 0000000000000180 R09: 9cd08d909b919a89

R10: ff26e458f74655c0 R11: 0000000000000000 R12: ff26e458f74655c0

R13: ff745a70c82539b0 R14: d0d0d0d0d0d0d0d0 R15: 2f2f2f2f2f2f2f2f

FS: 00007f5770896980(0000) GS:ff26e482c5d88000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f5764de50c0 CR3: 000000a72abb5001 CR4: 0000000000771ef0

PKRU: 55555554

Call Trace:

<TASK>

lookup_fast+0x9f/0x100

walk_component+0x1f/0x150

link_path_walk+0x20e/0x3d0

path_lookupat+0x68/0x180

filename_lookup+0xdc/0x1e0

vfs_statx+0x6c/0x140

vfs_fstatat+0x67/0xa0

__do_sys_newfstatat+0x24/0x60

do_syscall_64+0x6a/0x230

entry_SYSCALL_64_after_hwframe+0x76/0x7e

This is reachable with reused cached negative dentries. A Ceph lookup

or atomic_open can be handed a negative dentry that is already hashed,

and fs/ceph/dir.c then hits one of two paths that incorrectly assume

"negative" also means "unhashed":

- ceph_finish_lookup():

MDS reply is -ENOENT with no trace

-> d_add(dentry, NULL)

- ceph_lookup():

local ENOENT fast path for a complete directory with shared caps

-> d_add(dentry, NULL)

Both paths can therefore re-add an already-hashed negative dentry.

Ceph already uses the correct pattern elsewhere: ceph_fill_trace() only

calls d_add(dn, NULL) for a negative null-dentry reply when d_unhashed(dn)

is true.

Fix both fs/ceph/dir.c sites the same way: only call d_add() for a

negative dentry when it is actually unhashed. If the negative dentry

is already hashed, leave it in place and reuse it as-is.

This preserves the existing behavior for unhashed dentries while

avoiding d_hash list corruption for reused hashed negatives.