If a read races with an invalidation followed by another read, it is possible for a folio to be replaced with a higher-order folio. If that happens, we'll see a sibling entry for the new folio in the next iteration of the loop. This manifests as a NULL pointer dereference while holding the RCU read lock.
Handle this by simply returning. The next call will find the new folio and handle it correctly. The other ways of handling this rare race are more complex and it's just not worth it.
Reported-by: Dave Chinner david@fromorbit.com Reported-by: Brian Foster bfoster@redhat.com Debugged-by: Brian Foster bfoster@redhat.com Tested-by: Brian Foster bfoster@redhat.com Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") Cc: stable@vger.kernel.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org --- mm/filemap.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/mm/filemap.c b/mm/filemap.c index 577068868449..ffdfbc8b0e3c 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2385,6 +2385,8 @@ static void filemap_get_read_batch(struct address_space *mapping, continue; if (xas.xa_index > max || xa_is_value(folio)) break; + if (xa_is_sibling(folio)) + break; if (!folio_try_get_rcu(folio)) goto retry;
On Sun, Jun 19, 2022 at 04:11:42PM +0100, Matthew Wilcox (Oracle) wrote:
If a read races with an invalidation followed by another read, it is possible for a folio to be replaced with a higher-order folio. If that happens, we'll see a sibling entry for the new folio in the next iteration of the loop. This manifests as a NULL pointer dereference while holding the RCU read lock.
Handle this by simply returning. The next call will find the new folio and handle it correctly. The other ways of handling this rare race are more complex and it's just not worth it.
Reported-by: Dave Chinner david@fromorbit.com Reported-by: Brian Foster bfoster@redhat.com Debugged-by: Brian Foster bfoster@redhat.com Tested-by: Brian Foster bfoster@redhat.com Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") Cc: stable@vger.kernel.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org
This has survived my testing for several days now. I think I grok the fix and analysis that lead to it, so FWIW:
Reviewed-by: Brian Foster bfoster@redhat.com
mm/filemap.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/mm/filemap.c b/mm/filemap.c index 577068868449..ffdfbc8b0e3c 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2385,6 +2385,8 @@ static void filemap_get_read_batch(struct address_space *mapping, continue; if (xas.xa_index > max || xa_is_value(folio)) break;
if (xa_is_sibling(folio))break;2.35.1
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Matthew Wilcox (Oracle)