[syzbot] WARNING in hugetlb_change_protection

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e65c6a46df94 Merge tag 'drm-fixes-2022-07-30' of git://ano..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=139cc282080000
kernel config: https://syzkaller.appspot.com/x/.config?x=26034e6fe0075dad
dashboard link: https://syzkaller.appspot.com/bug?extid=824e71311e757a9689ff
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
userspace arch: i386

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

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+824e71...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 28745 at include/linux/swapops.h:319 make_pte_marker_entry include/linux/swapops.h:319 [inline]
WARNING: CPU: 1 PID: 28745 at include/linux/swapops.h:319 make_pte_marker include/linux/swapops.h:342 [inline]
WARNING: CPU: 1 PID: 28745 at include/linux/swapops.h:319 hugetlb_change_protection+0xf85/0x1610 mm/hugetlb.c:6392
Modules linked in:
CPU: 1 PID: 28745 Comm: syz-executor.3 Not tainted 5.19.0-rc8-syzkaller-00146-ge65c6a46df94 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022
RIP: 0010:make_pte_marker_entry include/linux/swapops.h:319 [inline]
RIP: 0010:make_pte_marker include/linux/swapops.h:342 [inline]
RIP: 0010:hugetlb_change_protection+0xf85/0x1610 mm/hugetlb.c:6392
Code: e8 d0 5a b7 ff 0f b6 94 24 80 00 00 00 48 8b 84 24 98 00 00 00 84 d2 0f 84 ef 02 00 00 49 89 c4 e9 48 fb ff ff e8 ab 5e b7 ff <0f> 0b 48 b9 00 00 00 00 00 fc ff df 48 89 d8 48 c1 e8 03 80 3c 08
RSP: 0018:ffffc90014cc7780 EFLAGS: 00010212
RAX: 000000000000082a RBX: ffff88807750e820 RCX: ffffc90006723000
RDX: 0000000000040000 RSI: ffffffff81c30c25 RDI: 0000000000000007
RBP: ffff888074de5ea0 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000004 R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000004 R15: ffff88801fcc8e00
FS: 0000000000000000(0000) GS:ffff8880b9b00000(0063) knlGS:00000000f7f06b40
CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020000040 CR3: 000000001b84c000 CR4: 00000000003526e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
change_protection+0x96b/0x3ad0 mm/mprotect.c:463
mwriteprotect_range+0x387/0x5c0 mm/userfaultfd.c:759
userfaultfd_writeprotect fs/userfaultfd.c:1823 [inline]
userfaultfd_ioctl+0x438/0x4340 fs/userfaultfd.c:1997
compat_ptr_ioctl+0x67/0x90 fs/ioctl.c:906
__do_compat_sys_ioctl+0x1c7/0x290 fs/ioctl.c:968
do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
__do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178
do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:203
entry_SYSENTER_compat_after_hwframe+0x70/0x82
RIP: 0023:0xf7f0b549
Code: 03 74 c0 01 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
RSP: 002b:00000000f7f065cc EFLAGS: 00000296 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000c018aa06
RDX: 00000000200000c0 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>
----------------
Code disassembly (best guess):
0: 03 74 c0 01 add 0x1(%rax,%rax,8),%esi
4: 10 05 03 74 b8 01 adc %al,0x1b87403(%rip) # 0x1b8740d
a: 10 06 adc %al,(%rsi)
c: 03 74 b4 01 add 0x1(%rsp,%rsi,4),%esi
10: 10 07 adc %al,(%rdi)
12: 03 74 b0 01 add 0x1(%rax,%rsi,4),%esi
16: 10 08 adc %cl,(%rax)
18: 03 74 d8 01 add 0x1(%rax,%rbx,8),%esi
1c: 00 00 add %al,(%rax)
1e: 00 00 add %al,(%rax)
20: 00 51 52 add %dl,0x52(%rcx)
23: 55 push %rbp
24: 89 e5 mov %esp,%ebp
26: 0f 34 sysenter
28: cd 80 int $0x80
* 2a: 5d pop %rbp <-- trapping instruction
2b: 5a pop %rdx
2c: 59 pop %rcx
2d: c3 retq
2e: 90 nop
2f: 90 nop
30: 90 nop
31: 90 nop
32: 8d b4 26 00 00 00 00 lea 0x0(%rsi,%riz,1),%esi
39: 8d b4 26 00 00 00 00 lea 0x0(%rsi,%riz,1),%esi


---
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[Mike Kravetz]

I'll start looking at this, but adding Peter this may be related to his
recent changes.
--
Mike Kravetz

[Peter Xu]

Hi, Mike,

Thanks for forwarding.

On Wed, Aug 03, 2022 at 10:02:37AM -0700, Mike Kravetz wrote:
> I'll start looking at this, but adding Peter this may be related to his
> recent changes.
> --
> Mike Kravetz
>
> On 08/03/22 08:32, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: e65c6a46df94 Merge tag 'drm-fixes-2022-07-30' of git://ano..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=139cc282080000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=26034e6fe0075dad
> > dashboard link: https://syzkaller.appspot.com/bug?extid=824e71311e757a9689ff
> > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > userspace arch: i386
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+824e71...@syzkaller.appspotmail.com
> >
> > ------------[ cut here ]------------
> > WARNING: CPU: 1 PID: 28745 at include/linux/swapops.h:319 make_pte_marker_entry include/linux/swapops.h:319 [inline]

This is the warning code I added to make sure pte marker won't be created
if not configured at all:

static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
{
/* This should never be called if !CONFIG_PTE_MARKER */
WARN_ON_ONCE(1);
return swp_entry(0, 0);
}

The stack below comes from a UFFDIO_WRITEPROTECT, however logically it
shouldn't really reach there - if with !PTE_MARKER then it must be with
!PTE_MARKER_UFFD_WP, then we should have returned "false" if hugetlb wanted
to register with uffd-wp:

static inline bool vma_can_userfault(struct vm_area_struct *vma,
unsigned long vm_flags)
{
if (vm_flags & VM_UFFD_MINOR)
return is_vm_hugetlb_page(vma) || vma_is_shmem(vma);

#ifndef CONFIG_PTE_MARKER_UFFD_WP
/*
* If user requested uffd-wp but not enabled pte markers for
* uffd-wp, then shmem & hugetlbfs are not supported but only
* anonymous.
*/
if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
return false;
#endif
return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
vma_is_shmem(vma);
}

Then the UFFDIO_WRITEPROTECT should have failed already.. at:

if (!userfaultfd_wp(dst_vma))
goto out_unlock;

in mwriteprotect_range().

I still have no idea how the bot managed to trigger a real wr-protect upon
this vma (which I don't think should have registered with uffd-wp but maybe
it can be worked around somehow..). However to make this even safer we can
also guard the pte marker callers with CONFIG_PTE_MARKER_UFFD_WP. Patch
attached for that, but since this seems to have no reproducer yet maybe no
easy way to verify it.

At the meantime, I'd also like to double check the kernel config to make
sure CONFIG_PTE_MARKER_UFFD_WP will always be unset when CONFIG_PTE_MARKER=n.
Anyone knows where I can fetch the config somewhere?

Thanks,

--
Peter Xu

[Dmitry Vyukov]

Hi Peter,

If you mean the kernel config for the kernel that produces the
WARNING, it's in the report above under the "kernel config" link.

Note that so far it was triggered only once:
https://syzkaller.appspot.com/bug?extid=824e71311e757a9689ff
which may suggest a very narrow race condition.

[Peter Xu]

Thanks Dmitry. That was a stupid question and Mike helped me to figure it
out.. Somehow I had an impression that config was normally attached to the
end of the email so I didn't try looking upwards. :(

>
> Note that so far it was triggered only once:
> https://syzkaller.appspot.com/bug?extid=824e71311e757a9689ff
> which may suggest a very narrow race condition.

I found that the config is as expected (PTE_MARKER_UFFD_WP not defined), so
far I still don't know how the race has happened, so maybe the patch
attachd can at least help avoid the warning. It'll be interesting to know
the race if hopefully it can have a reproducer somehow.

Thanks,

--
Peter Xu

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.