[syzbot] [mm?] general protection fault in hpage_collapse_scan_file

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8568bb2ccc27 Add linux-next specific files for 20240405
git tree: linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=152f4805180000
kernel config: https://syzkaller.appspot.com/x/.config?x=48ca5acf8d2eb3bc
dashboard link: https://syzkaller.appspot.com/bug?extid=57adb2a4b9d206521bc2
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=1268258d180000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1256598d180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1d120b5e779c/disk-8568bb2c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a89e3589a585/vmlinux-8568bb2c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/045e657c0e0d/bzImage-8568bb2c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+57adb2...@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 5080 Comm: syz-executor931 Not tainted 6.9.0-rc2-next-20240405-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
RIP: 0010:PageTail include/linux/page-flags.h:284 [inline]
RIP: 0010:const_folio_flags include/linux/page-flags.h:312 [inline]
RIP: 0010:folio_test_locked include/linux/page-flags.h:512 [inline]
RIP: 0010:collapse_file mm/khugepaged.c:1907 [inline]
RIP: 0010:hpage_collapse_scan_file+0x1ea3/0x63e0 mm/khugepaged.c:2292
Code: 48 8d bc 24 30 02 00 00 e8 9a a1 f9 ff 4c 8b bc 24 30 02 00 00 49 8d 5f 08 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 48 89 df e8 6f a1 f9 ff 48 8b 1b 48 89 de 48 83
RSP: 0018:ffffc9000340f420 EFLAGS: 00010247
RAX: 0000000000000000 RBX: 0000000000000006 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc9000340f7d0 R08: ffffffff81cb9212 R09: 1ffffffff1f5802d
R10: dffffc0000000000 R11: fffffbfff1f5802e R12: ffffc9000340f6b0
R13: 0000000000000000 R14: ffffc9000340f5f0 R15: fffffffffffffffe
FS: 000055558dc18380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000240 CR3: 000000006ee40000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
madvise_collapse+0x561/0xcc0 mm/khugepaged.c:2736
madvise_vma_behavior mm/madvise.c:1081 [inline]
madvise_walk_vmas mm/madvise.c:1255 [inline]
do_madvise+0xc3c/0x44a0 mm/madvise.c:1441
__do_sys_madvise mm/madvise.c:1456 [inline]
__se_sys_madvise mm/madvise.c:1454 [inline]
__x64_sys_madvise+0xa6/0xc0 mm/madvise.c:1454
do_syscall_64+0xfb/0x240
entry_SYSCALL_64_after_hwframe+0x72/0x7a
RIP: 0033:0x7f4cdb4292e9
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:00007fff99038cd8 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
RAX: ffffffffffffffda RBX: 00007fff99038eb8 RCX: 00007f4cdb4292e9
RDX: 0000000000000019 RSI: 0000000000600722 RDI: 0000000020000000
RBP: 00007f4cdb49c610 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fff99038ea8 R14: 0000000000000001 R15: 0000000000000001
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:PageTail include/linux/page-flags.h:284 [inline]
RIP: 0010:const_folio_flags include/linux/page-flags.h:312 [inline]
RIP: 0010:folio_test_locked include/linux/page-flags.h:512 [inline]
RIP: 0010:collapse_file mm/khugepaged.c:1907 [inline]
RIP: 0010:hpage_collapse_scan_file+0x1ea3/0x63e0 mm/khugepaged.c:2292
Code: 48 8d bc 24 30 02 00 00 e8 9a a1 f9 ff 4c 8b bc 24 30 02 00 00 49 8d 5f 08 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 48 89 df e8 6f a1 f9 ff 48 8b 1b 48 89 de 48 83
RSP: 0018:ffffc9000340f420 EFLAGS: 00010247
RAX: 0000000000000000 RBX: 0000000000000006 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc9000340f7d0 R08: ffffffff81cb9212 R09: 1ffffffff1f5802d
R10: dffffc0000000000 R11: fffffbfff1f5802e R12: ffffc9000340f6b0
R13: 0000000000000000 R14: ffffc9000340f5f0 R15: fffffffffffffffe
FS: 000055558dc18380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000240 CR3: 000000006ee40000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: 48 8d bc 24 30 02 00 lea 0x230(%rsp),%rdi
7: 00
8: e8 9a a1 f9 ff call 0xfff9a1a7
d: 4c 8b bc 24 30 02 00 mov 0x230(%rsp),%r15
14: 00
15: 49 8d 5f 08 lea 0x8(%r15),%rbx
19: 48 89 d8 mov %rbx,%rax
1c: 48 c1 e8 03 shr $0x3,%rax
20: 48 b9 00 00 00 00 00 movabs $0xdffffc0000000000,%rcx
27: fc ff df
* 2a: 80 3c 08 00 cmpb $0x0,(%rax,%rcx,1) <-- trapping instruction
2e: 74 08 je 0x38
30: 48 89 df mov %rbx,%rdi
33: e8 6f a1 f9 ff call 0xfff9a1a7
38: 48 8b 1b mov (%rbx),%rbx
3b: 48 89 de mov %rbx,%rsi
3e: 48 rex.W
3f: 83 .byte 0x83


---
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[Andrew Morton]

On Tue, 09 Apr 2024 03:16:20 -0700 syzbot <syzbot+57adb2...@syzkaller.appspotmail.com> wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 8568bb2ccc27 Add linux-next specific files for 20240405
> git tree: linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=152f4805180000
> kernel config: https://syzkaller.appspot.com/x/.config?x=48ca5acf8d2eb3bc
> dashboard link: https://syzkaller.appspot.com/bug?extid=57adb2a4b9d206521bc2
> 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=1268258d180000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1256598d180000

Help. From a quick look this seems to be claiming that collapse_file()
got to

VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);

with folio==NULL, but the code look solid regarding this.

Given that we have a reproducer, can we expect the bot to perform a
bisection for us?

[Zach O'Keefe]

I often don't see a successful automatic bisect, even with
reproducers. Hit or miss. I will take a closer look tomorrow -- the
reproducer doesn't look to be doing anything crazy.

Thanks,
Zach

[Zach O'Keefe]

I've only been able to reproduce this using the disk image provided by syzbot.

What is happening is we are calling MADV_COLLAPSE on an empty mapping
-- which actually reaches collapse_file() -> filemap_lock_folio()
after page_cache_sync_readahead() attempt. This of course fails
correctly, and I can see right before GPF that the returned pointer is
0xfffffffffffffffe, which is correctly ERR_PTR(-ENOENT). This should
be causing us to take the if (IS_ERR(folio)) {..} path .. but we
don't, and I don't know why. I haven't yet attempted to repro this
against other images. Will continue looking, but wanted to provide
some type of update -- even if it is a disappointing one -- so as to
not appear like I've disappeared.

Thanks,
Zach

> Thanks,
> Zach

[Zach O'Keefe]

Ugh. Was looking at the wrong source. Thanks hughd@ for mentioning
that IS_ERR(folio) changed recently, else I'd have spent more time on
it. Fixed by https://lore.kernel.org/all/ZhIWX8K0...@casper.infradead.org/

> Thanks,
> Zach
>
> > Thanks,
> > Zach