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

From: Jeongjun Park
Date: Sun Oct 06 2024 - 20:50:12 EST


Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> 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.

Oh, I see. After looking at the code again, I realized that
if we can get the folio, you should return a valid nr_pages
even if folio_trylock fails.

I'll send v2 patch with that shortly.

Regards,
Jeongjun Park