[PATCH] fs: fix overflow check in rw_verify_area()

[Matteo Croce <technoboy85@gmail.com>]

The overflow check in rw_verify_area() can itself overflow when
pos + count > LLONG_MAX, causing the sum to wrap to a negative value
and incorrectly return -EINVAL.

This can be reproduced easily by creating a 20 MB file and reading it
via splice() and a size of 0x7FFFFFFFFF000000. The syscall fails
when the file pos reaches 16 MB.

splice(3, NULL, 6, NULL, 9223372036837998592, 0) = 262144
splice(3, NULL, 6, NULL, 9223372036837998592, 0) = 262144
splice(3, NULL, 6, NULL, 9223372036837998592, 0) = -1 EINVAL (Invalid argument)

This can probably be triggered in other ways given that coreutils often
uses SSIZE_MAX as size argument[1][2]

[1] https://cgit.git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat.c?h=v9.9#n505
[2] https://cgit.git.savannah.gnu.org/cgit/coreutils.git/tree/src/copy-file-data.c?h=v9.9#n130
---
 fs/read_write.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 833bae068770..8cb4f5bba592 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -464,9 +464,13 @@ int rw_verify_area(int read_write, struct file *file, const loff_t *ppos, size_t
 				return -EINVAL;
 			if (count >= -pos) /* both values are in 0..LLONG_MAX */
 				return -EOVERFLOW;
-		} else if (unlikely((loff_t) (pos + count) < 0)) {
-			if (!unsigned_offsets(file))
-				return -EINVAL;
+		} else {
+			/* Clamp count to MAX_RW_COUNT for overflow check. */
+			loff_t end = min_t(loff_t, count, MAX_RW_COUNT);
+			if (unlikely(end > LLONG_MAX - pos)) {
+				if (!unsigned_offsets(file))
+					return -EINVAL;
+			}
 		}
 	}
 
-- 
2.52.0