Re: [syzbot] [ext4?] linux-next test error: general protection fault in __bh_submit

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

syzbot <syzbot+5a7a23af225c215902e7@syzkaller.appspotmail.com> writes:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    32b6ef9a5d0e Add linux-next specific files for G20260902
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=14b44125580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=32ceed1c94c11059
> dashboard link: https://syzkaller.appspot.com/bug?extid=5a7a23af225c215902e7
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/9a1f38fb6f24/disk-32b6ef9a.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f3152eed43a4/vmlinux-32b6ef9a.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e7fbd36e1b12/bzImage-32b6ef9a.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+5a7a23af225c215902e7@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: 4945 Comm: jbd2/sda1-8 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> RIP: 0010:const_folio_flags include/linux/page-flags.h:345 [inline]
> RIP: 0010:folio_test_dropbehind include/linux/page-flags.h:600 [inline]
> RIP: 0010:__bh_submit+0x22d/0x8d0 fs/buffer.c:1109
> Code: c1 e8 03 48 89 44 24 18 42 80 3c 20 00 74 08 48 89 df e8 26 f6 d7 ff 48 89 5c 24 28 48 8b 1b 4c 8d 7b 08 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 04 f6 d7 ff 4d 8b 3f 4c 89 fe 48
> RSP: 0018:ffffc9001016f868 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff84b96ff9
> RDX: 0000000000000001 RSI: 0000000000000004 RDI: ffff888075a7a660
> RBP: 0000000000009801 R08: ffff888075a7a6bf R09: 1ffff1100eb4f4d7
> R10: dffffc0000000000 R11: ffffed100eb4f4d8 R12: dffffc0000000000
> R13: ffff888075a7a640 R14: ffff8880123a17e0 R15: 0000000000000008
> FS:  0000000000000000(0000) GS:ffff888124ccb000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fe1ee627da0 CR3: 0000000074fa8000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  jbd2_journal_commit_transaction+0x22a9/0x5cd0 fs/jbd2/commit.c:751
>  kjournald2+0x3bc/0x750 fs/jbd2/journal.c:200
>  kthread+0x38b/0x480 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 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:const_folio_flags include/linux/page-flags.h:345 [inline]
> RIP: 0010:folio_test_dropbehind include/linux/page-flags.h:600 [inline]
> RIP: 0010:__bh_submit+0x22d/0x8d0 fs/buffer.c:1109
> Code: c1 e8 03 48 89 44 24 18 42 80 3c 20 00 74 08 48 89 df e8 26 f6 d7 ff 48 89 5c 24 28 48 8b 1b 4c 8d 7b 08 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 04 f6 d7 ff 4d 8b 3f 4c 89 fe 48
> RSP: 0018:ffffc9001016f868 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff84b96ff9
> RDX: 0000000000000001 RSI: 0000000000000004 RDI: ffff888075a7a660
> RBP: 0000000000009801 R08: ffff888075a7a6bf R09: 1ffff1100eb4f4d7
> R10: dffffc0000000000 R11: ffffed100eb4f4d8 R12: dffffc0000000000
> R13: ffff888075a7a640 R14: ffff8880123a17e0 R15: 0000000000000008
> FS:  0000000000000000(0000) GS:ffff888124dcb000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f87c696cf80 CR3: 000000000eb48000 CR4: 00000000003526f0
> ----------------
> Code disassembly (best guess):
>    0:	c1 e8 03             	shr    $0x3,%eax
>    3:	48 89 44 24 18       	mov    %rax,0x18(%rsp)
>    8:	42 80 3c 20 00       	cmpb   $0x0,(%rax,%r12,1)
>    d:	74 08                	je     0x17
>    f:	48 89 df             	mov    %rbx,%rdi
>   12:	e8 26 f6 d7 ff       	call   0xffd7f63d
>   17:	48 89 5c 24 28       	mov    %rbx,0x28(%rsp)
>   1c:	48 8b 1b             	mov    (%rbx),%rbx
>   1f:	4c 8d 7b 08          	lea    0x8(%rbx),%r15
>   23:	4c 89 f8             	mov    %r15,%rax
>   26:	48 c1 e8 03          	shr    $0x3,%rax
> * 2a:	42 80 3c 20 00       	cmpb   $0x0,(%rax,%r12,1) <-- trapping instruction
>   2f:	74 08                	je     0x39
>   31:	4c 89 ff             	mov    %r15,%rdi
>   34:	e8 04 f6 d7 ff       	call   0xffd7f63d
>   39:	4d 8b 3f             	mov    (%r15),%r15
>   3c:	4c 89 fe             	mov    %r15,%rsi
>   3f:	48                   	rex.W
>
>
> ---
> This report is generated by a bot. It may contain errors.
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>
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#syz test

diff --git a/fs/buffer.c b/fs/buffer.c
index 427d8a817cd5..f46fa6413032 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1106,7 +1106,8 @@ static void __bh_submit(struct buffer_head *bh, blk_opf_t opf,
 
 	bio = bio_alloc(bh->b_bdev, 1, opf, GFP_NOIO);
 
-	if (folio_test_dropbehind(bh->b_folio) && op_is_write(opf))
+	if (bh->b_folio && folio_test_dropbehind(bh->b_folio) &&
+	    op_is_write(opf))
 		bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
 
 	if (IS_ENABLED(CONFIG_FS_ENCRYPTION))