[syzbot] [block?] general protection fault in blk_mq_free_tags_callback

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 846bd2225ec3 Add linux-next specific files for 20250919
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13c238e2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=155e427c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17bb8142580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c53d48022f8a/disk-846bd222.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/483534e784c8/vmlinux-846bd222.xz
kernel image: https://storage.googleapis.com/syzbot-assets/721b36eec9b3/bzImage-846bd222.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5c5d41...@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 UID: 0 PID: 5962 Comm: kworker/0:3 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Workqueue: rcu_gp srcu_invoke_callbacks
RIP: 0010:__list_del_entry_valid_or_report+0x25/0x190 lib/list_debug.c:49
Code: 90 90 90 90 90 f3 0f 1e fa 41 57 41 56 41 55 41 54 53 48 89 fb 49 bd 00 00 00 00 00 fc ff df 48 83 c7 08 48 89 f8 48 c1 e8 03 <42> 80 3c 28 00 74 05 e8 af 58 47 fd 4c 8b 7b 08 48 89 d8 48 c1 e8
RSP: 0018:ffffc90003d2f8e8 EFLAGS: 00010202
RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffff88802f573c80
RDX: 0000000000000000 RSI: 0000000000000200 RDI: 0000000000000008
RBP: dffffc0000000000 R08: ffffffff8fe4db77 R09: 1ffffffff1fc9b6e
R10: dffffc0000000000 R11: ffffffff84b089f0 R12: 1ffff11004cb1e1f
R13: dffffc0000000000 R14: ffff88802658f0a0 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff8881257a2000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055946acdc950 CR3: 000000007a708000 CR4: 00000000003526f0
Call Trace:
<TASK>
__list_del_entry_valid include/linux/list.h:132 [inline]
__list_del_entry include/linux/list.h:223 [inline]
list_del_init include/linux/list.h:295 [inline]
blk_mq_free_tags_callback+0x5a/0x180 block/blk-mq-tag.c:593
srcu_invoke_callbacks+0x208/0x450 kernel/rcu/srcutree.c:1807
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__list_del_entry_valid_or_report+0x25/0x190 lib/list_debug.c:49
Code: 90 90 90 90 90 f3 0f 1e fa 41 57 41 56 41 55 41 54 53 48 89 fb 49 bd 00 00 00 00 00 fc ff df 48 83 c7 08 48 89 f8 48 c1 e8 03 <42> 80 3c 28 00 74 05 e8 af 58 47 fd 4c 8b 7b 08 48 89 d8 48 c1 e8
RSP: 0018:ffffc90003d2f8e8 EFLAGS: 00010202
RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffff88802f573c80
RDX: 0000000000000000 RSI: 0000000000000200 RDI: 0000000000000008
RBP: dffffc0000000000 R08: ffffffff8fe4db77 R09: 1ffffffff1fc9b6e
R10: dffffc0000000000 R11: ffffffff84b089f0 R12: 1ffff11004cb1e1f
R13: dffffc0000000000 R14: ffff88802658f0a0 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff8881257a2000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055946acdc950 CR3: 000000007a708000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
0: 90 nop
1: 90 nop
2: 90 nop
3: 90 nop
4: 90 nop
5: f3 0f 1e fa endbr64
9: 41 57 push %r15
b: 41 56 push %r14
d: 41 55 push %r13
f: 41 54 push %r12
11: 53 push %rbx
12: 48 89 fb mov %rdi,%rbx
15: 49 bd 00 00 00 00 00 movabs $0xdffffc0000000000,%r13
1c: fc ff df
1f: 48 83 c7 08 add $0x8,%rdi
23: 48 89 f8 mov %rdi,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
2f: 74 05 je 0x36
31: e8 af 58 47 fd call 0xfd4758e5
36: 4c 8b 7b 08 mov 0x8(%rbx),%r15
3a: 48 89 d8 mov %rbx,%rax
3d: 48 rex.W
3e: c1 .byte 0xc1
3f: e8 .byte 0xe8


---
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[Hillf Danton]

> Date: Mon, 22 Sep 2025 13:24:25 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 846bd2225ec3 Add linux-next specific files for 20250919
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=13c238e2580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
> dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=155e427c580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17bb8142580000

#syz test linux-next master

--- x/block/blk-mq-tag.c
+++ y/block/blk-mq-tag.c
@@ -565,6 +565,7 @@ struct blk_mq_tags *blk_mq_init_tags(uns
if (!tags)
return NULL;

+ INIT_LIST_HEAD(&tags->page_list);
tags->nr_tags = total_tags;
tags->nr_reserved_tags = reserved_tags;
spin_lock_init(&tags->lock);
--

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

arch/x86/kvm/emulate.c:4091:2: error: call to undeclared function 'F'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
arch/x86/kvm/emulate.c:4093:2: error: field designator cannot initialize a non-struct, non-union type 'u8[7]' (aka 'unsigned char[7]')
arch/x86/kvm/emulate.c:4094:2: error: field designator 'flags' does not refer to any field in type 'union (unnamed union at arch/x86/kvm/emulate.c:198:2)'
arch/x86/kvm/emulate.c:4094:2: error: field designator 'u' does not refer to any field in type 'union (unnamed union at arch/x86/kvm/emulate.c:198:2)'
arch/x86/kvm/emulate.c:4095:2: error: initialization of non-aggregate type 'int (*)(struct x86_emulate_ctxt *)' with a designated initializer list


Tested on:

commit: bf2602a3 Add linux-next specific files for 20250922
git tree: linux-next

kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=166138e2580000

[Hillf Danton]

> Date: Mon, 22 Sep 2025 13:24:25 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 846bd2225ec3 Add linux-next specific files for 20250919
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=13c238e2580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
> dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=155e427c580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17bb8142580000

#syz test linux-next master

--- x/block/blk-mq-tag.c
+++ y/block/blk-mq-tag.c
@@ -565,6 +565,7 @@ struct blk_mq_tags *blk_mq_init_tags(uns
if (!tags)
return NULL;

+ INIT_LIST_HEAD(&tags->page_list);
tags->nr_tags = total_tags;
tags->nr_reserved_tags = reserved_tags;
spin_lock_init(&tags->lock);

--- x/arch/x86/kvm/emulate.c
+++ y/arch/x86/kvm/emulate.c
@@ -3991,6 +3991,7 @@ static int check_perm_out(struct x86_emu
#define MD(_f, _m) { .flags = ((_f) | ModeDual), .u.mdual = (_m) }
#define E(_f, _e) { .flags = ((_f) | Escape | ModRM), .u.esc = (_e) }
#define I(_f, _e) { .flags = (_f), .u.execute = (_e) }
+#define F(_f, _e) { .flags = (_f) | Fastop, .u.fastop = (_e) }
#define II(_f, _e, _i) \
{ .flags = (_f)|Intercept, .u.execute = (_e), .intercept = x86_intercept_##_i }
#define IIP(_f, _e, _i, _p) \
--

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

arch/x86/kvm/emulate.c:4092:2: error: use of undeclared identifier 'Fastop'
arch/x86/kvm/emulate.c:4093:2: error: use of undeclared identifier 'Fastop'


Tested on:

commit: bf2602a3 Add linux-next specific files for 20250922
git tree: linux-next

kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=14143534580000

[Ming Lei]

The initialization is needed, otherwise the unitialized tags->page_list
may be accessed in the error code path of blk_mq_alloc_rq_map().

Thanks,

[Jens Axboe]

Adding Ming, looks related to the SRCU changes perhaps?

--
Jens Axboe

[Yu Kuai]

Hi,

A quick look, yes it's related. Root cause is that blk_mq_free_tags()
can be called after blk_mq_init_tags(), while tags->page_list is still
not initialized by blk_mq_alloc_rqs().

Thanks,
Kuai

[Hillf Danton]

> Date: Mon, 22 Sep 2025 13:24:25 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 846bd2225ec3 Add linux-next specific files for 20250919
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=13c238e2580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=135377594f35b576
> dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=155e427c580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17bb8142580000

#syz test linux-next master

--- x/block/blk-mq-tag.c
+++ y/block/blk-mq-tag.c
@@ -565,6 +565,7 @@ struct blk_mq_tags *blk_mq_init_tags(uns
if (!tags)
return NULL;

+ INIT_LIST_HEAD(&tags->page_list);
tags->nr_tags = total_tags;
tags->nr_reserved_tags = reserved_tags;
spin_lock_init(&tags->lock);

--- x/arch/x86/kvm/emulate.c
+++ y/arch/x86/kvm/emulate.c
@@ -4001,7 +4001,6 @@ static int check_perm_out(struct x86_emu
#define D2bv(_f) D((_f) | ByteOp), D(_f)
#define D2bvIP(_f, _i, _p) DIP((_f) | ByteOp, _i, _p), DIP(_f, _i, _p)
#define I2bv(_f, _e) I((_f) | ByteOp, _e), I(_f, _e)
-#define F2bv(_f, _e) F((_f) | ByteOp, _e), F(_f, _e)
#define I2bvIP(_f, _e, _i, _p) \
IIP((_f) | ByteOp, _e, _i, _p), IIP(_f, _e, _i, _p)

@@ -4088,8 +4087,8 @@ static const struct opcode group4[] = {
};

static const struct opcode group5[] = {
- F(DstMem | SrcNone | Lock, em_inc),
- F(DstMem | SrcNone | Lock, em_dec),
+ I(DstMem | SrcNone | Lock, em_inc),
+ I(DstMem | SrcNone | Lock, em_dec),
I(SrcMem | NearBranch | IsBranch | ShadowStack, em_call_near_abs),
I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack, em_call_far),
I(SrcMem | NearBranch | IsBranch, em_jmp_abs),
--

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+5c5d41...@syzkaller.appspotmail.com
Tested-by: syzbot+5c5d41...@syzkaller.appspotmail.com

Tested on:

commit: bf2602a3 Add linux-next specific files for 20250922
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=14b8627c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6707f92bf75f45f3

dashboard link: https://syzkaller.appspot.com/bug?extid=5c5d41e80248d610221f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=12534142580000

Note: testing is done by a robot and is best-effort only.