Date: Mon, 22 Apr 2024 12:07:42 +0200
From: David Hildenbrand <david@...hat.com>
To: syzbot <syzbot+d8426b591c36b21c750e@...kaller.appspotmail.com>,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 pasha.tatashin@...een.com, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in __page_table_check_ptes_set

On 21.04.24 22:16, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    4eab35893071 Add linux-next specific files for 20240417
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1727a61b180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=27920e47287645ff
> dashboard link: https://syzkaller.appspot.com/bug?extid=d8426b591c36b21c750e
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=156da22d180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=163dfec7180000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/9f7d6c097fb4/disk-4eab3589.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/287b16352982/vmlinux-4eab3589.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/23839c65c573/bzImage-4eab3589.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d8426b591c36b21c750e@...kaller.appspotmail.com
> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 5084 at mm/page_table_check.c:199 __page_table_check_pte mm/page_table_check.c:199 [inline]
> WARNING: CPU: 0 PID: 5084 at mm/page_table_check.c:199 __page_table_check_ptes_set+0x1db/0x420 

I think this is

if (pte_present(pte) && pte_uffd_wp(pte))
	WARN_ON_ONCE(pte_write(pte));

mm/page_table_check.c:213
> Modules linked in:
> CPU: 0 PID: 5084 Comm: syz-executor382 Not tainted 6.9.0-rc4-next-20240417-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
> RIP: 0010:__page_table_check_pte mm/page_table_check.c:199 [inline]
> RIP: 0010:__page_table_check_ptes_set+0x1db/0x420 mm/page_table_check.c:213
> Code: 48 8b 7c 24 40 48 c7 c6 80 19 46 8e e8 ee df 8e ff 41 83 fc 1d 74 18 41 83 fc 1a 75 1d e8 5d da 8e ff eb 10 e8 56 da 8e ff 90 <0f> 0b 90 eb 10 e8 4b da 8e ff 90 0f 0b 90 eb 05 e8 40 da 8e ff 48
> RSP: 0018:ffffc9000366f740 EFLAGS: 00010293
> RAX: ffffffff8207833a RBX: ffffc9000366f7c0 RCX: ffff888022af3c00
> RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000000
> RBP: ffffc9000366f830 R08: ffffffff820782af R09: 1ffffd40000a6a10
> R10: dffffc0000000000 R11: fffff940000a6a11 R12: 0000000000000000
> R13: 0000000014d42c67 R14: 0000000000000001 R15: 0000000000000000
> FS:  0000555567f79380(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000000066c7e0 CR3: 0000000078cb0000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   <TASK>
>   page_table_check_ptes_set include/linux/page_table_check.h:74 [inline]
>   set_ptes include/linux/pgtable.h:267 [inline]
>   __ptep_modify_prot_commit include/linux/pgtable.h:1269 [inline]
>   ptep_modify_prot_commit include/linux/pgtable.h:1302 [inline]
>   change_pte_range mm/mprotect.c:194 [inline]
>   change_pmd_range mm/mprotect.c:424 [inline]
>   change_pud_range mm/mprotect.c:457 [inline]
>   change_p4d_range mm/mprotect.c:480 [inline]
>   change_protection_range mm/mprotect.c:508 [inline]
>   change_protection+0x2770/0x3cc0 mm/mprotect.c:542
>   mprotect_fixup+0x740/0xa90 mm/mprotect.c:655
>   do_mprotect_pkey+0x90d/0xe00 mm/mprotect.c:820
>   __do_sys_mprotect mm/mprotect.c:841 [inline]
>   __se_sys_mprotect mm/mprotect.c:838 [inline]
>   __x64_sys_mprotect+0x80/0x90 mm/mprotect.c:838
>   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>   do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f45514bf429
> Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 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 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffe52191598 EFLAGS: 00000246 ORIG_RAX: 000000000000000a
> RAX: ffffffffffffffda RBX: 00007ffe52191768 RCX: 00007f45514bf429
> RDX: 000000000000000f RSI: 0000000000004000 RDI: 0000000020ffc000
> RBP: 00007f4551532610 R08: 00007ffe52191768 R09: 00007ffe52191768
> R10: 00007ffe52191768 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007ffe52191758 R14: 0000000000000001 R15: 0000000000000001
>   </TASK>

Did we find a real issue that involves mprotect()?

At least can_change_pte_writable() should always return "false" for 
userfaultfd_pte_wp().

Do we maybe have a uffd-wp PTE outside of a UFFD_WP VMA?

Or was the PTE already writable and we only detect it now as we call 
mprotect()? (missed to detect it earlier?)

> 
> 
> ---
> 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 syzkaller@...glegroups.com.
> 
> syzbot will keep track of this issue. See:
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> 
> If the report is already addressed, let syzbot know by replying with:
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> 
> If you want syzbot to run the reproducer, reply with:
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> If you attach or paste a git patch, syzbot will apply it before testing.
> 
> If you want to overwrite report's subsystems, reply with:
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> (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
> 

-- 
Cheers,

David / dhildenb

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