linux-kernel - Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry

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Date:   Wed, 26 Jan 2022 11:51:36 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Yang Shi <shy828301@...il.com>, kirill.shutemov@...ux.intel.com,
        jannh@...gle.com, willy@...radead.org, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration
 entry

On 20.01.22 21:28, Yang Shi wrote:
> The syzbot reported the below BUG:
> 
> kernel BUG at include/linux/page-flags.h:785!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 4392 Comm: syz-executor560 Not tainted 5.16.0-rc6-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  page_mapcount include/linux/mm.h:837 [inline]
>  smaps_account+0x470/0xb10 fs/proc/task_mmu.c:466
>  smaps_pte_entry fs/proc/task_mmu.c:538 [inline]
>  smaps_pte_range+0x611/0x1250 fs/proc/task_mmu.c:601
>  walk_pmd_range mm/pagewalk.c:128 [inline]
>  walk_pud_range mm/pagewalk.c:205 [inline]
>  walk_p4d_range mm/pagewalk.c:240 [inline]
>  walk_pgd_range mm/pagewalk.c:277 [inline]
>  __walk_page_range+0xe23/0x1ea0 mm/pagewalk.c:379
>  walk_page_vma+0x277/0x350 mm/pagewalk.c:530
>  smap_gather_stats.part.0+0x148/0x260 fs/proc/task_mmu.c:768
>  smap_gather_stats fs/proc/task_mmu.c:741 [inline]
>  show_smap+0xc6/0x440 fs/proc/task_mmu.c:822
>  seq_read_iter+0xbb0/0x1240 fs/seq_file.c:272
>  seq_read+0x3e0/0x5b0 fs/seq_file.c:162
>  vfs_read+0x1b5/0x600 fs/read_write.c:479
>  ksys_read+0x12d/0x250 fs/read_write.c:619
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7faa2af6c969
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 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:00007faa2aefd288 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00007faa2aff4418 RCX: 00007faa2af6c969
> RDX: 0000000000002025 RSI: 0000000020000100 RDI: 0000000000000003
> RBP: 00007faa2aff4410 R08: 00007faa2aefd700 R09: 0000000000000000
> R10: 00007faa2aefd700 R11: 0000000000000246 R12: 00007faa2afc20ac
> R13: 00007fff7e6632bf R14: 00007faa2aefd400 R15: 0000000000022000
>  </TASK>
> Modules linked in:
> ---[ end trace 24ec93ff95e4ac3d ]---
> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 

Does this point at the bigger issue that reading the mapcount without
having the page locked is completely unstable?


-- 
Thanks,

David / dhildenb