Re: kernel BUG in page_add_anon_rmap

[Matthew Wilcox]

On Wed, Jan 25, 2023 at 11:59:16PM +0000, Sanan Hasanov wrote:
> Good day, dear maintainers,
> 
> We found a bug using a modified kernel configuration file used by syzbot.
> 
> We enhanced the coverage of the configuration file using our tool, klocalizer.
> 
> Kernel Branch: 6.2.0-rc5-next-20230124
> Kernel config: https://drive.google.com/file/d/1MZSgIF4R9QfikEuF5siUIZVPce-GiJQK/view?usp=sharing
> Reproducer: https://drive.google.com/file/d/1H5KWkT9VVMWTUVVgIaZi6J-fmukRx-BM/view?usp=sharing
> 
> Thank you!
> 
> Best regards,
> Sanan Hasanov
> 
> head: 0000000000020000 0000000000000000 00000004ffffffff ffff8881002b8000
> page dumped because: VM_BUG_ON_PAGE(!first && (flags & (( rmap_t)((((1UL))) << (0)))))
> ------------[ cut here ]------------

I know it says "cut here" and you did that, but including just a few
lines above that would be so much more helpful.  I can infer that this
is a multi-page folio, but more than that is hard to tell.

> kernel BUG at mm/rmap.c:1248!

That tracks with VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page);

> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 7 PID: 14932 Comm: syz-executor.1 Not tainted 6.2.0-rc5-next-20230124 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> RIP: 0010:page_add_anon_rmap+0xddd/0x11c0 mm/rmap.c:1248
> Code: c0 ff 48 8b 34 24 48 89 df e8 1f ff 07 00 49 89 c6 e9 85 f6 ff ff e8 52 73 c0 ff 48 c7 c6 c0 3c d8 89 48 89 ef e8 b3 23 f8 ff <0f> 0b e8 3c 73 c0 ff 48 c7 c6 00 3b d8 89 48 89 ef e8 9d 23 f8 ff
> RSP: 0018:ffffc9000c56f7b0 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffff88807efc6f30 RCX: 0000000000000000
> RDX: ffff8880464fd7c0 RSI: ffffffff81be733d RDI: fffff520018adedb
> RBP: ffffea0000c68080 R08: 0000000000000056 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000001 R12: ffffea0000c68000
> R13: 0000000000000001 R14: ffffea0000c68088 R15: 0000000000000000
> FS:  00007f717898a700(0000) GS:ffff888119f80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7178947d78 CR3: 000000004a9e6000 CR4: 0000000000350ee0
> Call Trace:
>  <TASK>
>  remove_migration_pte+0xaa6/0x1390 mm/migrate.c:261

                        if (folio_test_anon(folio))
                                page_add_anon_rmap(new, vma, pvmw.address,
                                                   rmap_flags);

Earlier in that function, we had:
                if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
                        rmap_flags |= RMAP_EXCLUSIVE;

so that also makes sense.  We can also infer that RMAP_COMPOUND wasn't
set, so we're trying to do just one page from the folio.

All right, back to rmap.c:

                first = atomic_inc_and_test(&page->_mapcount);

So first is clearly false (ie _mapcount was not -1), implying somebody
else already mapped this page.  Not really sure what's going on at
this point.  Seems unlikely that the folio changes in
remove_migration_pte() are responsible since they're from last January.
Huang has some more changes to migrate.c that I don't feel qualified
to judge.

Nothing's jumping out at me as obviously wrong.  Is it possible to
do a bisect?

>  rmap_walk_anon+0x23a/0x5b0 mm/rmap.c:2433
>  rmap_walk+0x96/0xd0 mm/rmap.c:2509
>  remove_migration_ptes mm/migrate.c:294 [inline]
>  migrate_folio_move mm/migrate.c:1307 [inline]
>  migrate_pages_batch+0x15f8/0x3910 mm/migrate.c:1809
>  migrate_pages+0x1b56/0x22e0 mm/migrate.c:1961
>  do_mbind mm/mempolicy.c:1329 [inline]
>  kernel_mbind+0x4c0/0x790 mm/mempolicy.c:1476
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x39/0x80 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f717788edcd
> Code: 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa 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:00007f7178989bf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ed
> RAX: ffffffffffffffda RBX: 00007f71779bc050 RCX: 00007f717788edcd
> RDX: 0000000000000004 RSI: 0000000000001000 RDI: 0000000020002000
> RBP: 00007f71778fc59c R08: 00000000000007ff R09: 0000000000000002
> R10: 00000000200000c0 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ffd2c012f6f R14: 00007ffd2c013110 R15: 00007f7178989d80
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:page_add_anon_rmap+0xddd/0x11c0 mm/rmap.c:1248
> Code: c0 ff 48 8b 34 24 48 89 df e8 1f ff 07 00 49 89 c6 e9 85 f6 ff ff e8 52 73 c0 ff 48 c7 c6 c0 3c d8 89 48 89 ef e8 b3 23 f8 ff <0f> 0b e8 3c 73 c0 ff 48 c7 c6 00 3b d8 89 48 89 ef e8 9d 23 f8 ff
> RSP: 0018:ffffc9000c56f7b0 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffff88807efc6f30 RCX: 0000000000000000
> RDX: ffff8880464fd7c0 RSI: ffffffff81be733d RDI: fffff520018adedb
> RBP: ffffea0000c68080 R08: 0000000000000056 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000001 R12: ffffea0000c68000
> R13: 0000000000000001 R14: ffffea0000c68088 R15: 0000000000000000
> FS:  00007f717898a700(0000) GS:ffff888119f80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7178947d78 CR3: 000000004a9e6000 CR4: 0000000000350ee0
>