NULL Pointer Dereference Affecting kernel-rt-64k-devel-matched package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt-64k-devel-matched package and not the kernel-rt-64k-devel-matched package as distributed by RHEL. See How to fix? for RHEL:9 relevant fixed versions and status.

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

rbd: fix null-ptr-deref when device_add_disk() fails

do_rbd_add() publishes the device with device_add() before calling device_add_disk(). If device_add_disk() fails after device_add() succeeds, the error path calls rbd_free_disk() directly and then later falls through to rbd_dev_device_release(), which calls rbd_free_disk() again. This double teardown can leave blk-mq cleanup operating on invalid state and trigger a null-ptr-deref in __blk_mq_free_map_and_rqs(), reached from blk_mq_free_tag_set().

Fix this by following the normal remove ordering: call device_del() before rbd_dev_device_release() when device_add_disk() fails after device_add(). That keeps the teardown sequence consistent and avoids re-entering disk cleanup through the wrong path.

The bug was first flagged by an experimental analysis tool we are developing for kernel memory-management bugs while analyzing v6.13-rc1. The tool is still under development and is not yet publicly available.

We reproduced the bug on v7.0 with a real Ceph backend and a QEMU x86_64 guest booted with KASAN and CONFIG_FAILSLAB enabled. The reproducer confines failslab injections to the __add_disk() range and injects fail-nth while mapping an RBD image through /sys/bus/rbd/add_single_major.

On the unpatched kernel, fail-nth=4 reliably triggered the fault:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 273 Comm: bash Not tainted 7.0.0-01247-gd60bc1401583 #6 PREEMPT(lazy)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
RIP: 0010:__blk_mq_free_map_and_rqs+0x8c/0x240
Code: 00 00 48 8b 6b 60 41 89 f4 49 c1 e4 03 4c 01 e5 45 85 ed 0f 85 0a 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 e9 48 c1 e9 03 <80> 3c 01 00 0f 85 31 01 00 00 4c 8b 6d 00 4d 85 ed 0f 84 e2 00 00
RSP: 0018:ff1100000ab0fac8 EFLAGS: 00000246
RAX: dffffc0000000000 RBX: ff1100000c4806a0 RCX: 0000000000000000
RDX: 0000000000000002 RSI: 0000000000000000 RDI: ff1100000c4806f4
RBP: 0000000000000000 R08: 0000000000000001 R09: ffe21c000189001b
R10: ff1100000c4800df R11: ff1100006cf37be0 R12: 0000000000000000
R13: 0000000000000000 R14: ff1100000c480700 R15: ff1100000c480004
FS:  00007f0fbe8fe740(0000) GS:ff110000e5851000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe53473b2e0 CR3: 0000000012eef000 CR4: 00000000007516f0
PKRU: 55555554
Call Trace:
 <TASK>
 blk_mq_free_tag_set+0x77/0x460
 do_rbd_add+0x1446/0x2b80
 ? __pfx_do_rbd_add+0x10/0x10
 ? lock_acquire+0x18c/0x300
 ? find_held_lock+0x2b/0x80
 ? sysfs_file_kobj+0xb6/0x1b0
 ? __pfx_sysfs_kf_write+0x10/0x10
 kernfs_fop_write_iter+0x2f4/0x4a0
 vfs_write+0x98e/0x1000
 ? expand_files+0x51f/0x850
 ? __pfx_vfs_write+0x10/0x10
 ksys_write+0xf2/0x1d0
 ? __pfx_ksys_write+0x10/0x10
 do_syscall_64+0x115/0x690
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f0fbea15907
Code: 10 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
RSP: 002b:00007ffe22346ea8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000058 RCX: 00007f0fbea15907
RDX: 0000000000000058 RSI: 0000563ace6c0ef0 RDI: 0000000000000001
RBP: 0000563ace6c0ef0 R08: 0000563ace6c0ef0 R09: 6b6435726d694141
R10: 5250337279762f78 R11: 0000000000000246 R12: 0000000000000058
R13: 00007f0fbeb1c780 R14: ff1100000c480700 R15: ff1100000c480004
 </TASK>

With this fix applied, rerunning the reproducer over fail-nth=1..256 yields no KASAN reports.

[ idryomov: rename err_out_device_del -> err_out_device ]

• https://access.redhat.com/security/cve/CVE-2026-46079
• https://git.kernel.org/stable/c/059fb7656723c1b77c2fc0e64b7aa99d6bb65e8e
• https://git.kernel.org/stable/c/2f4809a879f0750c7790bbeeae86c9505797a06f
• https://git.kernel.org/stable/c/564cd8f4aeb9a938e470c5c91922fd02e4d41acc
• https://git.kernel.org/stable/c/ad0126ffcba8777109852979eaaa6dca6703abdb
• https://git.kernel.org/stable/c/d1fef92e414433ca7b89abf85cb0df42b8d475eb
• https://git.kernel.org/stable/c/78bd0c143dea4b7a4c23c13356987ca0eafb442e