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@xxxxxxxxxxxxxxxxxxxxxxxxx
__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
