io_uring/memmap: cast nr_pages to size_t before shifting

JIRA: https://issues.redhat.com/browse/RHEL-106845

commit 33503c0
Author: Jens Axboe <axboe@kernel.dk>
Date:   Fri Aug 8 06:35:14 2025 -0600

    io_uring/memmap: cast nr_pages to size_t before shifting

    If the allocated size exceeds UINT_MAX, then it's necessary to cast
    the mr->nr_pages value to size_t to prevent it from overflowing. In
    practice this isn't much of a concern as the required memory size will
    have been validated upfront, and accounted to the user. And > 4GB sizes
    will be necessary to make the lack of a cast a problem, which greatly
    exceeds normal user locked_vm settings that are generally in the kb to
    mb range. However, if root is used, then accounting isn't done, and
    then it's possible to hit this issue.

    Link: https://lore.kernel.org/all/6895b298.050a0220.7f033.0059.GAE@google.com/
    Cc: stable@vger.kernel.org
    Reported-by: syzbot+23727438116feb13df15@syzkaller.appspotmail.com
    Fixes: 087f997 ("io_uring/memmap: implement mmap for regions")
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

io_uring/memmap.c

@@ -155,7 +155,7 @@ static int io_region_allocate_pages(struct io_ring_ctx *ctx,
 				    unsigned long mmap_offset)
 {
 	gfp_t gfp = GFP_KERNEL_ACCOUNT | __GFP_ZERO | __GFP_NOWARN;
-	unsigned long size = mr->nr_pages << PAGE_SHIFT;
+	size_t size = (size_t) mr->nr_pages << PAGE_SHIFT;
 	unsigned long nr_allocated;
 	struct page **pages;
 	void *p;