linux-kernel - Re: [syzbot] KCSAN: data-race in flush_tlb_batched_pending / try_to_unmap

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Date:   Fri, 19 Nov 2021 22:11:10 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Marco Elver <elver@...gle.com>,
        "Huang, Ying" <ying.huang@...el.com>
Cc:     syzbot <syzbot+aa5bebed695edaccf0df@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, syzkaller-bugs@...glegroups.com,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [syzbot] KCSAN: data-race in flush_tlb_batched_pending /
 try_to_unmap_one


> On Nov 18, 2021, at 6:49 AM, Marco Elver <elver@...gle.com> wrote:
> 
> On Thu, Nov 18, 2021 at 06:20AM -0800, syzbot wrote:
>> Hello,
>> 
>> syzbot found the following issue on:
>> 
>> HEAD commit:    42eb8fdac2fc Merge tag 'gfs2-v5.16-rc2-fixes' of git://git..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13160026b00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=a70237460d215073
>> dashboard link: https://syzkaller.appspot.com/bug?extid=aa5bebed695edaccf0df
>> compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2
>> 
>> 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+aa5bebed695edaccf0df@...kaller.appspotmail.com
>> 
>> ==================================================================
>> BUG: KCSAN: data-race in flush_tlb_batched_pending / try_to_unmap_one
>> 
>> write to 0xffff8881072cfbbc of 1 bytes by task 17406 on cpu 1:
>> flush_tlb_batched_pending+0x5f/0x80 mm/rmap.c:691
>> madvise_free_pte_range+0xee/0x7d0 mm/madvise.c:594
>> 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+0x981/0x1160 mm/pagewalk.c:379
>> walk_page_range+0x131/0x300 mm/pagewalk.c:475
>> madvise_free_single_vma mm/madvise.c:734 [inline]
>> madvise_dontneed_free mm/madvise.c:822 [inline]
>> madvise_vma mm/madvise.c:996 [inline]
>> do_madvise+0xe4a/0x1140 mm/madvise.c:1202
>> __do_sys_madvise mm/madvise.c:1228 [inline]
>> __se_sys_madvise mm/madvise.c:1226 [inline]
>> __x64_sys_madvise+0x5d/0x70 mm/madvise.c:1226
>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>> do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>> entry_SYSCALL_64_after_hwframe+0x44/0xae
>> 
>> write to 0xffff8881072cfbbc of 1 bytes by task 71 on cpu 0:
>> set_tlb_ubc_flush_pending mm/rmap.c:636 [inline]
>> try_to_unmap_one+0x60e/0x1220 mm/rmap.c:1515
>> rmap_walk_anon+0x2fb/0x470 mm/rmap.c:2301
>> try_to_unmap+0xec/0x110
>> shrink_page_list+0xe91/0x2620 mm/vmscan.c:1719
>> shrink_inactive_list+0x3fb/0x730 mm/vmscan.c:2394
>> shrink_list mm/vmscan.c:2621 [inline]
>> shrink_lruvec+0x3c9/0x710 mm/vmscan.c:2940
>> shrink_node_memcgs+0x23e/0x410 mm/vmscan.c:3129
>> shrink_node+0x8f6/0x1190 mm/vmscan.c:3252
>> kswapd_shrink_node mm/vmscan.c:4022 [inline]
>> balance_pgdat+0x702/0xd30 mm/vmscan.c:4213
>> kswapd+0x200/0x340 mm/vmscan.c:4473
>> kthread+0x2c7/0x2e0 kernel/kthread.c:327
>> ret_from_fork+0x1f/0x30
>> 
>> value changed: 0x01 -> 0x00
>> 
>> Reported by Kernel Concurrency Sanitizer on:
>> CPU: 0 PID: 71 Comm: kswapd0 Not tainted 5.16.0-rc1-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> ==================================================================
> 
> Reading 3ea277194daae, I can't quite determine if this is safe and
> expected.
> 
> Per this observed write/write race, depending on interleaving
> tlb_flush_batched can randomly be true or false after
> set_tlb_ubc_flush_pending().
> 
> Is this safe?
> 

I do not think it is safe and I am not the only one.

https://lore.kernel.org/linux-mm/20210131001132.3368247-21-namit@vmware.com/

Perhaps Huang has the cycles to fix it.

Regards,
Nadav