Re: [PATCH] mm: migrate: fix data-race in migrate_folio_unmap()

[David Hildenbrand <david@redhat.com>]

On 22.09.24 17:17, Jeongjun Park wrote:
> I found a report from syzbot [1]
> 
> When __folio_test_movable() is called in migrate_folio_unmap() to read
> folio->mapping, a data race occurs because the folio is read without
> protecting it with folio_lock.
> 
> This can cause unintended behavior because folio->mapping is initialized
> to a NULL value. Therefore, I think it is appropriate to call
> __folio_test_movable() under the protection of folio_lock to prevent
> data-race.
> 

We hold a folio reference, would we really see PAGE_MAPPING_MOVABLE 
flip? Hmm

Even a racing __ClearPageMovable() would still leave 
PAGE_MAPPING_MOVABLE set.

> [1]
> 
> ==================================================================
> BUG: KCSAN: data-race in __filemap_remove_folio / migrate_pages_batch
> 
> write to 0xffffea0004b81dd8 of 8 bytes by task 6348 on cpu 0:
>   page_cache_delete mm/filemap.c:153 [inline]
>   __filemap_remove_folio+0x1ac/0x2c0 mm/filemap.c:233
>   filemap_remove_folio+0x6b/0x1f0 mm/filemap.c:265
>   truncate_inode_folio+0x42/0x50 mm/truncate.c:178
>   shmem_undo_range+0x25b/0xa70 mm/shmem.c:1028
>   shmem_truncate_range mm/shmem.c:1144 [inline]
>   shmem_evict_inode+0x14d/0x530 mm/shmem.c:1272
>   evict+0x2f0/0x580 fs/inode.c:731
>   iput_final fs/inode.c:1883 [inline]
>   iput+0x42a/0x5b0 fs/inode.c:1909
>   dentry_unlink_inode+0x24f/0x260 fs/dcache.c:412
>   __dentry_kill+0x18b/0x4c0 fs/dcache.c:615
>   dput+0x5c/0xd0 fs/dcache.c:857
>   __fput+0x3fb/0x6d0 fs/file_table.c:439
>   ____fput+0x1c/0x30 fs/file_table.c:459
>   task_work_run+0x13a/0x1a0 kernel/task_work.c:228
>   resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>   exit_to_user_mode_loop kernel/entry/common.c:114 [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+0xbe/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 0xffffea0004b81dd8 of 8 bytes by task 6342 on cpu 1:
>   __folio_test_movable include/linux/page-flags.h:699 [inline]
>   migrate_folio_unmap mm/migrate.c:1199 [inline]
>   migrate_pages_batch+0x24c/0x1940 mm/migrate.c:1797
>   migrate_pages_sync mm/migrate.c:1963 [inline]
>   migrate_pages+0xff1/0x1820 mm/migrate.c:2072
>   do_mbind mm/mempolicy.c:1390 [inline]
>   kernel_mbind mm/mempolicy.c:1533 [inline]
>   __do_sys_mbind mm/mempolicy.c:1607 [inline]
>   __se_sys_mbind+0xf76/0x1160 mm/mempolicy.c:1603
>   __x64_sys_mbind+0x78/0x90 mm/mempolicy.c:1603
>   x64_sys_call+0x2b4d/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:238
>   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: 0xffff888127601078 -> 0x0000000000000000

Note that this doesn't flip PAGE_MAPPING_MOVABLE, just some unrelated bits.

> 
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Cc: stable@vger.kernel.org
> Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> ---
>   mm/migrate.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..e62dac12406b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1118,7 +1118,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>   	int rc = -EAGAIN;
>   	int old_page_state = 0;
>   	struct anon_vma *anon_vma = NULL;
> -	bool is_lru = !__folio_test_movable(src);
> +	bool is_lru;
>   	bool locked = false;
>   	bool dst_locked = false;
>   
> @@ -1172,6 +1172,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>   	locked = true;
>   	if (folio_test_mlocked(src))
>   		old_page_state |= PAGE_WAS_MLOCKED;
> +	is_lru = !__folio_test_movable(src);


Looks straight forward, though

Acked-by: David Hildenbrand <david@redhat.com>


-- 
Cheers,

David / dhildenb