[syzbot] [fs?] kernel BUG in kpagecount_read

[syzbot] [fs?] kernel BUG in kpagecount_read

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    c061cf420ded Merge tag 'trace-v6.13-rc3' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11ee22df980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c22efbd20f8da769
dashboard link: https://syzkaller.appspot.com/bug?extid=3d7dc5eaba6b932f8535
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/565ec42c1d1a/disk-c061cf42.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/142d1c3a6f99/vmlinux-c061cf42.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b21efab0a38b/bzImage-c061cf42.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com

 __napi_poll.constprop.0+0xb7/0x550 net/core/dev.c:6883
 napi_poll net/core/dev.c:6952 [inline]
 net_rx_action+0xa94/0x1010 net/core/dev.c:7074
 handle_softirqs+0x213/0x8f0 kernel/softirq.c:561
 __do_softirq kernel/softirq.c:595 [inline]
 invoke_softirq kernel/softirq.c:435 [inline]
 __irq_exit_rcu+0x109/0x170 kernel/softirq.c:662
 irq_exit_rcu+0x9/0x30 kernel/softirq.c:678
 common_interrupt+0xbf/0xe0 arch/x86/kernel/irq.c:278
 asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:693
------------[ cut here ]------------
kernel BUG at ./include/linux/mm.h:1221!
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 1 UID: 0 PID: 11868 Comm: syz.3.1633 Tainted: G     U             6.13.0-rc3-syzkaller-00062-gc061cf420ded #0
Tainted: [U]=USER
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/25/2024
RIP: 0010:folio_entire_mapcount include/linux/mm.h:1221 [inline]
RIP: 0010:folio_precise_page_mapcount fs/proc/internal.h:172 [inline]
RIP: 0010:kpagecount_read+0x477/0x570 fs/proc/page.c:71
Code: 31 ff 49 29 c4 48 8b 44 24 08 4c 01 20 e8 41 77 61 ff eb 92 e8 ca 74 61 ff 48 8b 3c 24 48 c7 c6 20 eb 61 8b e8 6a 34 a8 ff 90 <0f> 0b 4c 89 ff e8 ef de c3 ff e9 5a ff ff ff e8 a5 74 61 ff 48 8b
RSP: 0018:ffffc9000ca57ca0 EFLAGS: 00010246
RAX: 0000000000080000 RBX: 0000000000000000 RCX: ffffc9000d719000
RDX: 0000000000080000 RSI: ffffffff8237da36 RDI: ffff888035460444
RBP: 0000000001058a80 R08: 0000000000000001 R09: fffffbfff2d36daf
R10: ffffffff969b6d7f R11: 0000000000000004 R12: 00000000201a9000
R13: 0000000000034eb0 R14: dffffc0000000000 R15: 0000000000000000
FS:  00007f56afbaa6c0(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002669000 CR3: 000000006b460000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 pde_read fs/proc/inode.c:308 [inline]
 proc_reg_read+0x11d/0x330 fs/proc/inode.c:318
 vfs_read+0x1df/0xbe0 fs/read_write.c:563
 ksys_read+0x12b/0x250 fs/read_write.c:708
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f56aed85d29
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f56afbaa038 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007f56aef75fa0 RCX: 00007f56aed85d29
RDX: 00000000fffffea1 RSI: 0000000020001a80 RDI: 0000000000000005
RBP: 00007f56aee01a20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f56aef75fa0 R15: 00007ffc1f509908
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:folio_entire_mapcount include/linux/mm.h:1221 [inline]
RIP: 0010:folio_precise_page_mapcount fs/proc/internal.h:172 [inline]
RIP: 0010:kpagecount_read+0x477/0x570 fs/proc/page.c:71
Code: 31 ff 49 29 c4 48 8b 44 24 08 4c 01 20 e8 41 77 61 ff eb 92 e8 ca 74 61 ff 48 8b 3c 24 48 c7 c6 20 eb 61 8b e8 6a 34 a8 ff 90 <0f> 0b 4c 89 ff e8 ef de c3 ff e9 5a ff ff ff e8 a5 74 61 ff 48 8b
RSP: 0018:ffffc9000ca57ca0 EFLAGS: 00010246
RAX: 0000000000080000 RBX: 0000000000000000 RCX: ffffc9000d719000
RDX: 0000000000080000 RSI: ffffffff8237da36 RDI: ffff888035460444
RBP: 0000000001058a80 R08: 0000000000000001 R09: fffffbfff2d36daf
R10: ffffffff969b6d7f R11: 0000000000000004 R12: 00000000201a9000
R13: 0000000000034eb0 R14: dffffc0000000000 R15: 0000000000000000
FS:  00007f56afbaa6c0(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000370e000 CR3: 000000006b460000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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Re: [syzbot] [fs?] kernel BUG in kpagecount_read

[David Hildenbrand]

On 10.01.25 15:33, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    c061cf420ded Merge tag 'trace-v6.13-rc3' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11ee22df980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c22efbd20f8da769
> dashboard link: https://syzkaller.appspot.com/bug?extid=3d7dc5eaba6b932f8535
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/565ec42c1d1a/disk-c061cf42.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/142d1c3a6f99/vmlinux-c061cf42.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/b21efab0a38b/bzImage-c061cf42.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
> 
>   __napi_poll.constprop.0+0xb7/0x550 net/core/dev.c:6883
>   napi_poll net/core/dev.c:6952 [inline]
>   net_rx_action+0xa94/0x1010 net/core/dev.c:7074
>   handle_softirqs+0x213/0x8f0 kernel/softirq.c:561
>   __do_softirq kernel/softirq.c:595 [inline]
>   invoke_softirq kernel/softirq.c:435 [inline]
>   __irq_exit_rcu+0x109/0x170 kernel/softirq.c:662
>   irq_exit_rcu+0x9/0x30 kernel/softirq.c:678
>   common_interrupt+0xbf/0xe0 arch/x86/kernel/irq.c:278
>   asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:693
> ------------[ cut here ]------------
> kernel BUG at ./include/linux/mm.h:1221!
> Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
> CPU: 1 UID: 0 PID: 11868 Comm: syz.3.1633 Tainted: G     U             6.13.0-rc3-syzkaller-00062-gc061cf420ded #0
> Tainted: [U]=USER
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/25/2024
> RIP: 0010:folio_entire_mapcount include/linux/mm.h:1221 [inline]
> RIP: 0010:folio_precise_page_mapcount fs/proc/internal.h:172 [inline]
> RIP: 0010:kpagecount_read+0x477/0x570 fs/proc/page.c:71
> Code: 31 ff 49 29 c4 48 8b 44 24 08 4c 01 20 e8 41 77 61 ff eb 92 e8 ca 74 61 ff 48 8b 3c 24 48 c7 c6 20 eb 61 8b e8 6a 34 a8 ff 90 <0f> 0b 4c 89 ff e8 ef de c3 ff e9 5a ff ff ff e8 a5 74 61 ff 48 8b

Kind of known (at least to me :) ).

We race with splitting the large folio and end up in
	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);

We could take a speculative reference on the folio to prevent the 
concurrent split ... but I am not really sure it is worth it.

-- 
Cheers,

David / dhildenb

Re: [syzbot] [fs?] kernel BUG in kpagecount_read

[David Hildenbrand]

On 10.01.25 16:09, David Hildenbrand wrote:
> On 10.01.25 15:33, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    c061cf420ded Merge tag 'trace-v6.13-rc3' of git://git.kern..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=11ee22df980000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=c22efbd20f8da769
>> dashboard link: https://syzkaller.appspot.com/bug?extid=3d7dc5eaba6b932f8535
>> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> Downloadable assets:
>> disk image: https://storage.googleapis.com/syzbot-assets/565ec42c1d1a/disk-c061cf42.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/142d1c3a6f99/vmlinux-c061cf42.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/b21efab0a38b/bzImage-c061cf42.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+3d7dc5eaba6b932f8535@syzkaller.appspotmail.com
>>
>>    __napi_poll.constprop.0+0xb7/0x550 net/core/dev.c:6883
>>    napi_poll net/core/dev.c:6952 [inline]
>>    net_rx_action+0xa94/0x1010 net/core/dev.c:7074
>>    handle_softirqs+0x213/0x8f0 kernel/softirq.c:561
>>    __do_softirq kernel/softirq.c:595 [inline]
>>    invoke_softirq kernel/softirq.c:435 [inline]
>>    __irq_exit_rcu+0x109/0x170 kernel/softirq.c:662
>>    irq_exit_rcu+0x9/0x30 kernel/softirq.c:678
>>    common_interrupt+0xbf/0xe0 arch/x86/kernel/irq.c:278
>>    asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:693
>> ------------[ cut here ]------------
>> kernel BUG at ./include/linux/mm.h:1221!
>> Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
>> CPU: 1 UID: 0 PID: 11868 Comm: syz.3.1633 Tainted: G     U             6.13.0-rc3-syzkaller-00062-gc061cf420ded #0
>> Tainted: [U]=USER
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/25/2024
>> RIP: 0010:folio_entire_mapcount include/linux/mm.h:1221 [inline]
>> RIP: 0010:folio_precise_page_mapcount fs/proc/internal.h:172 [inline]
>> RIP: 0010:kpagecount_read+0x477/0x570 fs/proc/page.c:71
>> Code: 31 ff 49 29 c4 48 8b 44 24 08 4c 01 20 e8 41 77 61 ff eb 92 e8 ca 74 61 ff 48 8b 3c 24 48 c7 c6 20 eb 61 8b e8 6a 34 a8 ff 90 <0f> 0b 4c 89 ff e8 ef de c3 ff e9 5a ff ff ff e8 a5 74 61 ff 48 8b
> 
> Kind of known (at least to me :) ).
> 
> We race with splitting the large folio and end up in
> 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> 
> We could take a speculative reference on the folio to prevent the
> concurrent split ... but I am not really sure it is worth it.
> 

The following should work:

diff --git a/fs/proc/page.c b/fs/proc/page.c
index a55f5acefa974..2868248ffccf6 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -67,10 +67,17 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
                  * memmaps that were actually initialized.
                  */
                 page = pfn_to_online_page(pfn);
-               if (page)
-                       mapcount = folio_precise_page_mapcount(page_folio(page),
-                                                              page);
+               if (!page)
+                       goto write_mapcount;
  
+               folio = page_folio(page);
+               if (!folio_try_get(folio))
+                       goto write_mapcount;
+               if (page_folio(page) == folio)
+                       mapcount = folio_precise_page_mapcount(folio, page);
+               folio_put(folio);
+
+write_mapcount:
                 if (put_user(mapcount, out)) {
                         ret = -EFAULT;
                         break;


In general, I dislike interfaces that can take references
on arbitrary folios. But well, at least this is only accessible to root.

-- 
Cheers,

David / dhildenb