[syzbot] [fs?] kernel BUG in kpagecount_read

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: [email protected]

__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


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at [email protected].

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

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

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.