[PATCH v3 2/8] mm: userfaultfd: check for start + len overflow in validate_range

[Axel Rasmussen <axelrasmussen@google.com>]

Most userfaultfd ioctls take a `start + len` range as an argument.
We have the validate_range helper to check that such ranges are valid.
However, some (but not all!) ioctls *also* check that `start + len`
doesn't wrap around (overflow).

Just check for this in validate_range. This saves some repetitive code,
and adds the check to some ioctls which weren't bothering to check for
it before.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 fs/userfaultfd.c | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 7cecd49e078b..2e84684c46f0 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1306,6 +1306,8 @@ static __always_inline int validate_range(struct mm_struct *mm,
 		return -EINVAL;
 	if (len > task_size - start)
 		return -EINVAL;
+	if (start + len <= start)
+		return -EINVAL;
 	return 0;
 }
 
@@ -1760,14 +1762,8 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
 	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
 	if (ret)
 		goto out;
-	/*
-	 * double check for wraparound just in case. copy_from_user()
-	 * will later check uffdio_copy.src + uffdio_copy.len to fit
-	 * in the userland range.
-	 */
+
 	ret = -EINVAL;
-	if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
-		goto out;
 	if (uffdio_copy.mode & ~(UFFDIO_COPY_MODE_DONTWAKE|UFFDIO_COPY_MODE_WP))
 		goto out;
 	if (uffdio_copy.mode & UFFDIO_COPY_MODE_WP)
@@ -1927,11 +1923,6 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
 		goto out;
 
 	ret = -EINVAL;
-	/* double check for wraparound just in case. */
-	if (uffdio_continue.range.start + uffdio_continue.range.len <=
-	    uffdio_continue.range.start) {
-		goto out;
-	}
 	if (uffdio_continue.mode & ~(UFFDIO_CONTINUE_MODE_DONTWAKE |
 				     UFFDIO_CONTINUE_MODE_WP))
 		goto out;
-- 
2.41.0.255.g8b1d071c50-goog