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@xxxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 7e2a5e5ab217 ("mm: migrate: use __folio_test_movable()")
> > Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx>
>
>
> --
> Cheers,
>
> David / dhildenb
>
