Re: [PATCH v2] mm: swap: prevent possible data-race in __try_to_reclaim_swap

[Kairui Song]

On Mon, Oct 14, 2024 at 10:17 AM Jeongjun Park <aha310510@xxxxxxxxx> wrote:
> > Kairui Song <ryncsn@xxxxxxxxx> wrote:
> >
> > On Mon, Oct 7, 2024 at 3:06 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 folio->entry
> >> 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 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 | 7 ++++---
> >> 1 file changed, 4 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index 0cded32414a1..eb782fcd5627 100644
> >> --- a/mm/swapfile.c
> >> +++ b/mm/swapfile.c
> >> @@ -194,9 +194,6 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> >>        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;
> >>
> >> @@ -210,6 +207,10 @@ 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);
> >> +
> >>        need_reclaim = ((flags & TTRS_ANYWAY) ||
> >>                        ((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
> >>                        ((flags & TTRS_FULL) && mem_cgroup_swap_full(folio)));
> >> --
> >
> > Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > Hi Andrew,
> >
> > Will this be added to stable and 6.12? 862590ac3708 is already in 6.12
> > and this fixes a potential issue of it.
>
> As far as I can see, commit 862590ac3708 was applied starting
> from 6.12-rc1, so it looks like no additional commits are needed
> for the stable version.

Hi, sorry for the confusion, I meant mm-stable, not the stable branch.
It's better to merge this in 6.12.

> Regards,
>
> Jeongjun Park