Re: [PATCH] mm: swap: prevent possible data-race in __try_to_reclaim_swap

[Kairui Song]

On Fri, Oct 4, 2024 at 10:26 PM Jeongjun Park <aha310510@xxxxxxxxx> wrote:
>
> A report [1] was uploaded from syzbot.
>
> In the previous commit 862590ac3708 ("mm: swap: allow cache reclaim to skip
> slot cache"), the __try_to_reclaim_swap() function reads offset and nr_pages
> from folio without folio_lock protection.
>
> In the currently reported KCSAN log, it is assumed that the actual data-race
> will not occur because the calltrace that does WRITE already obtains the
> folio_lock and then writes.
>
> However, the existing __try_to_reclaim_swap() function was already implemented
> to perform reads under folio_lock protection [1], and there is a risk of a
> data-race occurring through a function other than the one shown in the KCSAN
> log.
>
> Therefore, I think it is appropriate to change all read operations for
> folio to be performed under folio_lock.
>
> [1]
>
> ==================================================================
> BUG: KCSAN: data-race in __delete_from_swap_cache / __try_to_reclaim_swap
>
> write to 0xffffea0004c90328 of 8 bytes by task 5186 on cpu 0:
>  __delete_from_swap_cache+0x1f0/0x290 mm/swap_state.c:163
>  delete_from_swap_cache+0x72/0xe0 mm/swap_state.c:243
>  folio_free_swap+0x1d8/0x1f0 mm/swapfile.c:1850
>  free_swap_cache mm/swap_state.c:293 [inline]
>  free_pages_and_swap_cache+0x1fc/0x410 mm/swap_state.c:325
>  __tlb_batch_free_encoded_pages mm/mmu_gather.c:136 [inline]
>  tlb_batch_pages_flush mm/mmu_gather.c:149 [inline]
>  tlb_flush_mmu_free mm/mmu_gather.c:366 [inline]
>  tlb_flush_mmu+0x2cf/0x440 mm/mmu_gather.c:373
>  zap_pte_range mm/memory.c:1700 [inline]
>  zap_pmd_range mm/memory.c:1739 [inline]
>  zap_pud_range mm/memory.c:1768 [inline]
>  zap_p4d_range mm/memory.c:1789 [inline]
>  unmap_page_range+0x1f3c/0x22d0 mm/memory.c:1810
>  unmap_single_vma+0x142/0x1d0 mm/memory.c:1856
>  unmap_vmas+0x18d/0x2b0 mm/memory.c:1900
>  exit_mmap+0x18a/0x690 mm/mmap.c:1864
>  __mmput+0x28/0x1b0 kernel/fork.c:1347
>  mmput+0x4c/0x60 kernel/fork.c:1369
>  exit_mm+0xe4/0x190 kernel/exit.c:571
>  do_exit+0x55e/0x17f0 kernel/exit.c:926
>  do_group_exit+0x102/0x150 kernel/exit.c:1088
>  get_signal+0xf2a/0x1070 kernel/signal.c:2917
>  arch_do_signal_or_restart+0x95/0x4b0 arch/x86/kernel/signal.c:337
>  exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
>  exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
>  syscall_exit_to_user_mode+0x59/0x130 kernel/entry/common.c:218
>  do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffffea0004c90328 of 8 bytes by task 5189 on cpu 1:
>  __try_to_reclaim_swap+0x9d/0x510 mm/swapfile.c:198
>  free_swap_and_cache_nr+0x45d/0x8a0 mm/swapfile.c:1915
>  zap_pte_range mm/memory.c:1656 [inline]
>  zap_pmd_range mm/memory.c:1739 [inline]
>  zap_pud_range mm/memory.c:1768 [inline]
>  zap_p4d_range mm/memory.c:1789 [inline]
>  unmap_page_range+0xcf8/0x22d0 mm/memory.c:1810
>  unmap_single_vma+0x142/0x1d0 mm/memory.c:1856
>  unmap_vmas+0x18d/0x2b0 mm/memory.c:1900
>  exit_mmap+0x18a/0x690 mm/mmap.c:1864
>  __mmput+0x28/0x1b0 kernel/fork.c:1347
>  mmput+0x4c/0x60 kernel/fork.c:1369
>  exit_mm+0xe4/0x190 kernel/exit.c:571
>  do_exit+0x55e/0x17f0 kernel/exit.c:926
>  __do_sys_exit kernel/exit.c:1055 [inline]
>  __se_sys_exit kernel/exit.c:1053 [inline]
>  __x64_sys_exit+0x1f/0x20 kernel/exit.c:1053
>  x64_sys_call+0x2d46/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:61
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> value changed: 0x0000000000000242 -> 0x0000000000000000
>
> Reported-by: syzbot+fa43f1b63e3aa6f66329@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 862590ac3708 ("mm: swap: allow cache reclaim to skip slot cache")
> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
> ---
>  mm/swapfile.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 0cded32414a1..904c21256fc2 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -193,13 +193,6 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
>         folio = filemap_get_folio(address_space, swap_cache_index(entry));
>         if (IS_ERR(folio))
>                 return 0;
> -
> -       /* offset could point to the middle of a large folio */
> -       entry = folio->swap;
> -       offset = swp_offset(entry);
> -       nr_pages = folio_nr_pages(folio);
> -       ret = -nr_pages;
> -
>         /*
>          * When this function is called from scan_swap_map_slots() and it's
>          * called by vmscan.c at reclaiming folios. So we hold a folio lock
> @@ -210,6 +203,12 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
>         if (!folio_trylock(folio))
>                 goto out;
>
> +       /* offset could point to the middle of a large folio */
> +       entry = folio->swap;
> +       offset = swp_offset(entry);
> +       nr_pages = folio_nr_pages(folio);
> +       ret = -nr_pages;
> +
>         need_reclaim = ((flags & TTRS_ANYWAY) ||
>                         ((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
>                         ((flags & TTRS_FULL) && mem_cgroup_swap_full(folio)));
> --
>

Thanks for catching this!

This could lead to real problems, holding reference is not enough for
protecting folio->swap. There are several BUG_ONs later that will be
triggered if it changed.

But you still have to keep `nr_pages ` and `ret` before the
`folio_trylock `, or `ret` will be uninitialized if folio_trylock
fails, this function should always return the page number even if the
try lock failed. And as WIlly said, `folio_nr_pages` doesn't require
folio lock.