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

[Matthew Wilcox <willy@infradead.org>]

On Mon, Sep 23, 2024 at 05:56:40PM +0200, David Hildenbrand wrote:
> 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

No; this shows a page cache folio getting truncated.  It's fine; really
a false alarm from the tool.  I don't think the proposed patch
introduces any problems, but it's all a bit meh.

> 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
>