Re: [syzbot] [block?] [ext4?] BUG: unable to handle kernel paging request in blk_mq_exit_hctx

[syzbot <syzbot+5307e3bcacec2ce4c2f0@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    6628e7d4c6b2 Linux 7.4-rc1
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=148f6add580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a58b6b4d667e7a61
dashboard link: https://syzkaller.appspot.com/bug?extid=5307e3bcacec2ce4c2f0
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
userspace arch: arm64
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1436344e580000

AI-generated root cause summary:
When dm_setup_md_queue() fails after add_disk() (for instance during
bd_link_disk_holder() or dm_sysfs_init()), the error path invokes del_gendisk(),
which calls blk_mq_exit_queue() -> blk_mq_exit_hctx(), freeing each hardware
context flush queue and setting hctx->fq = NULL. However, md->type remains
DM_TYPE_NONE and the mapped device is not torn down. When userspace retries
table_load on the device, dm_setup_md_queue() and dm_mq_init_request_queue() run
again, invoking blk_mq_init_allocated_queue() on the existing queue. During
__blk_mq_realloc_hw_ctxs(), blk_mq_exit_hctx() is called on the existing
hardware contexts, unconditionally dereferencing hctx->fq->flush_rq (offset
0x78) when hctx->fq is NULL, causing a kernel null-pointer dereference crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=5307e3bcacec2ce4c2f0

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5307e3bcacec2ce4c2f0@syzkaller.appspotmail.com

kobject: kobject_add_internal failed for dm (error: -12 parent: dm-9)
device-mapper: ioctl: unable to set up device queue for new table.
Unable to handle kernel paging request at virtual address dfff80000000000f
KASAN: null-ptr-deref in range [0x0000000000000078-0x000000000000007f]
Mem abort info:
  ESR = 0x0000000096000005
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x05: level 1 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
  CM = 0, WnR = 0, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0
[dfff80000000000f] address between user and kernel address ranges
Internal error: Oops: 0000000096000005 [#1]  SMP
Modules linked in:
CPU: 0 UID: 0 PID: 2579 Comm: syz.5.600 Not tainted syzkaller #0 PREEMPT 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/23/2026
pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : blk_mq_exit_hctx+0x64/0x498 block/blk-mq.c:3974
lr : blk_mq_exit_hctx+0x40/0x498 block/blk-mq.c:3973
sp : ffff800097b275c0
x29: ffff800097b275d0 x28: 0000000000000000 x27: 1fffe0001ae5652a
x26: dfff800000000000 x25: ffff0000d72b2950 x24: ffff0000e0621480
x23: 0000000000000078 x22: ffff0000d22e9400 x21: 0000000000000001
x20: ffff0000d72b2800 x19: ffff0000ca1f5388 x18: ffff800123625000
x17: ffff0001ace95387 x16: ffff0000d7a11d94 x15: 0000000000000001
x14: 00000000004bc250 x13: 0000000000000001 x12: 0000000000000003
x11: 0000000000000000 x10: ffff0000ca1f53bc x9 : 0000000000000000
x8 : 000000000000000f x7 : ffff800081826454 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000001 x3 : 0000000000000000
x2 : ffff0000d72b2800 x1 : ffff0000d7a11d80 x0 : 0000000000000000
Call trace:
 blk_mq_exit_hctx+0x64/0x498 block/blk-mq.c:3974 (P)
 __blk_mq_realloc_hw_ctxs+0x1f0/0x548 block/blk-mq.c:4612
 blk_mq_realloc_hw_ctxs block/blk-mq.c:4652 [inline]
 blk_mq_init_allocated_queue+0x2dc/0xe9c block/blk-mq.c:4694
 dm_mq_init_request_queue+0x2d8/0x474 drivers/md/dm-rq.c:573
 dm_setup_md_queue+0x134/0x434 drivers/md/dm.c:2611
 table_load+0x3d4/0x764 drivers/md/dm-ioctl.c:1661
 ctl_ioctl drivers/md/dm-ioctl.c:2221 [inline]
 dm_ctl_ioctl+0x884/0xbbc drivers/md/dm-ioctl.c:2243
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __arm64_sys_ioctl+0x154/0x1cc fs/ioctl.c:583
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0xec/0x23c arch/arm64/kernel/syscall.c:121
 do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
 el0_svc+0x60/0x1d8 arch/arm64/kernel/entry-common.c:796
 el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:815
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:606
Code: 97cf39bd f9400328 9101e117 d343fee8 (387a6908) 
---[ end trace 0000000000000000 ]---
----------------
Code disassembly (best guess):
   0:	97cf39bd 	bl	0xffffffffff3ce6f4
   4:	f9400328 	ldr	x8, [x25]
   8:	9101e117 	add	x23, x8, #0x78
   c:	d343fee8 	lsr	x8, x23, #3
* 10:	387a6908 	ldrb	w8, [x8, x26] <-- trapping instruction


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