[PATCH] mm/rmap: fix signedness bug in make_device_exclusive_range()

From: Dan Carpenter <dan.carpenter@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Alistair Popple <apopple@nvidia.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-mm@kvack.org, kernel-janitors@vger.kernel.org
Subject: [PATCH] mm/rmap: fix signedness bug in make_device_exclusive_range()
Date: Tue, 22 Jun 2021 22:03:00 +0300	[thread overview]
Message-ID: <YNIz5NVnZ5GiZ3u1@mwanda> (raw)

The get_user_pages_remote() function returns a long type, but we are
using "unsigned long i;" as the list iterator.  If "npages" is -ENOMEM,
the comparison "i < npages" is type promoted and "npages" becomes a very
high positive value.  The loop will then iterate until the kernel
crashes.

There are two ways to fix this.  Declare "i" as a long type or add an
explicit check for get_user_pages_remote() error returns.  Either
approach will work so let's do both.

Fixes: fa1e686e5f53 ("mm: device exclusive memory access")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
 mm/rmap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index e5210dde0c4d..fb5c59b95826 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2187,11 +2187,14 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
 				void *owner)
 {
 	long npages = (end - start) >> PAGE_SHIFT;
-	unsigned long i;
+	long i;
 
 	npages = get_user_pages_remote(mm, start, npages,
 				       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
 				       pages, NULL, NULL);
+	if (npages < 0)
+		return npages;
+
 	for (i = 0; i < npages; i++, start += PAGE_SIZE) {
 		if (!trylock_page(pages[i])) {
 			put_page(pages[i]);
-- 
2.30.2

