linux-kernel - Re: [stable-5.15 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry

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Date:   Thu, 17 Feb 2022 19:56:51 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     adobriyan@...il.com, akpm@...ux-foundation.org, david@...hat.com,
        jannh@...gle.com, kirill.shutemov@...ux.intel.com,
        nathan@...nel.org, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [stable-5.15 PATCH] fs/proc: task_mmu.c: don't read mapcount for
 migration entry

On Tue, Feb 15, 2022 at 02:15:03PM -0800, Yang Shi wrote:
> commit 24d7275ce2791829953ed4e72f68277ceb2571c6 upstream
> 
> 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
>   Call Trace:
>     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
> 
> The reproducer was trying to read /proc/$PID/smaps when calling
> MADV_FREE at the mean time.  MADV_FREE may split THPs if it is called
> for partial THP.  It may trigger the below race:
> 
>            CPU A                         CPU B
>            -----                         -----
>   smaps walk:                      MADV_FREE:
>   page_mapcount()
>     PageCompound()
>                                    split_huge_page()
>     page = compound_head(page)
>     PageDoubleMap(page)
> 
> When calling PageDoubleMap() this page is not a tail page of THP anymore
> so the BUG is triggered.
> 
> This could be fixed by elevated refcount of the page before calling
> mapcount, but that would prevent it from counting migration entries, and
> it seems overkilling because the race just could happen when PMD is
> split so all PTE entries of tail pages are actually migration entries,
> and smaps_account() does treat migration entries as mapcount == 1 as
> Kirill pointed out.
> 
> Add a new parameter for smaps_account() to tell this entry is migration
> entry then skip calling page_mapcount().  Don't skip getting mapcount
> for device private entries since they do track references with mapcount.
> 
> Pagemap also has the similar issue although it was not reported.  Fixed
> it as well.
> 
> [shy828301@...il.com: v4]
>   Link: https://lkml.kernel.org/r/20220203182641.824731-1-shy828301@gmail.com
> [nathan@...nel.org: avoid unused variable warning in pagemap_pmd_range()]
>   Link: https://lkml.kernel.org/r/20220207171049.1102239-1-nathan@kernel.org
> Link: https://lkml.kernel.org/r/20220120202805.3369-1-shy828301@gmail.com
> Fixes: e9b61f19858a ("thp: reintroduce split_huge_page()")
> Signed-off-by: Yang Shi <shy828301@...il.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> Reported-by: syzbot+1f52b3a18d5633fa7f82@...kaller.appspotmail.com
> Acked-by: David Hildenbrand <david@...hat.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Alexey Dobriyan <adobriyan@...il.com>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> ---
>  fs/proc/task_mmu.c | 40 +++++++++++++++++++++++++++++++---------
>  1 file changed, 31 insertions(+), 9 deletions(-)


Both backports applied, thanks.

greg k-h