linux-kernel - [PATCH 5.18 033/181] filemap: Handle sibling entries in filemap_get_read

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Message-Id: <20220627111945.522910063@linuxfoundation.org>
Date:   Mon, 27 Jun 2022 13:20:06 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dave Chinner <david@...morbit.com>,
        Brian Foster <bfoster@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>
Subject: [PATCH 5.18 033/181] filemap: Handle sibling entries in filemap_get_read_batch()

From: Matthew Wilcox (Oracle) <willy@...radead.org>

commit cb995f4eeba9d268fd4b56c2423ad6c1d1ea1b82 upstream.

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@...morbit.com>
Reported-by: Brian Foster <bfoster@...hat.com>
Debugged-by: Brian Foster <bfoster@...hat.com>
Tested-by: Brian Foster <bfoster@...hat.com>
Reviewed-by: Brian Foster <bfoster@...hat.com>
Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Cc: stable@...r.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 mm/filemap.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2385,6 +2385,8 @@ static void filemap_get_read_batch(struc
 			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;