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Date:   Sat, 19 Dec 2020 17:04:33 +0000
Message-Id: <20201219170433.2418867-1-kaleshsingh@google.com>
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Subject: [PATCH] mm: mremap - Fix extent calculation
From:   Kalesh Singh <kaleshsingh@google.com>
Cc:     surenb@google.com, minchan@kernel.org, joelaf@google.com,
        lokeshgidra@google.com, kaleshsingh@google.com,
        kernel-team@android.com, linux@roeck-us.net,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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When `next < old_addr`, `next - old_addr` arithmetic underflows
causing `extent` to be incorrect.

Make `extent` the smaller of `next - old_addr` or `old_end - old_addr`.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 mm/mremap.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index c5590afe7165..f554320281cc 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -358,7 +358,9 @@ static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
 
 	next = (old_addr + size) & mask;
 	/* even if next overflowed, extent below will be ok */
-	extent = (next > old_end) ? old_end - old_addr : next - old_addr;
+	extent = next - old_addr;
+	if (extent > old_end - old_addr)
+		extent = old_end - old_addr;
 	next = (new_addr + size) & mask;
 	if (extent > next - new_addr)
 		extent = next - new_addr;
-- 
2.29.2.729.g45daf8777d-goog