[PATCH] mm/hmm/test: Reject overflowing page counts

[Dan Carpenter <error27@gmail.com>]

The number of pages comes directly from userspace.  Shifting a value
larger than ULONG_MAX >> PAGE_SHIFT discards its high bits before the
existing end-address check.  A request for a huge number of pages can
therefore be accepted and processed as a much smaller request.

Reject page counts that cannot be represented as a byte size before
performing the shift.

So far as ChatGPT and I can tell this doesn't cause an issue in
practice but preventing this integer overflow is the correct thing to
do.

Fixes: b2ef9f5a5cb3 ("mm/hmm/test: add selftest driver for HMM")
Assisted-by: ChatGPT:gpt-5
Signed-off-by: Dan Carpenter <error27@gmail.com>
---
 lib/test_hmm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 6911daa9f854..b409c42bfe6f 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -1624,6 +1624,8 @@ static long dmirror_fops_unlocked_ioctl(struct file *filp,
 
 	if (cmd.addr & ~PAGE_MASK)
 		return -EINVAL;
+	if (cmd.npages > ULONG_MAX >> PAGE_SHIFT)
+		return -EINVAL;
 	if (cmd.addr >= (cmd.addr + (cmd.npages << PAGE_SHIFT)))
 		return -EINVAL;
 
-- 
2.53.0