[PATCH 5.15.y] mm/gup: check for overflow when converting nr_pages to bytes in GUP-fast

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit 21a3888d3d2f87b7f15ccea1f0befd57346fe899 ]

The overflow check in gup_fast_fallback() only covers the addition to
start. The preceding nr_pages << PAGE_SHIFT can already have truncated
the length on 32bit.

Users such as RapidIO's rio_dma_transfer() can derive such page counts
from a 64-bit userspace length.

Let's just check_shl_overflow() to reject any such overflow with
-EOVERFLOW early.

Fixes: c28b1fc70390 ("mm/gup: reorganize internal_get_user_pages_fast()")
Cc: stable@vger.kernel.org
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Barry Song <baohua@kernel.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-22-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: adapted the overflow check to internal_get_user_pages_fast(), changing its existing failure return from 0 to -EOVERFLOW ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/gup.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index c1508281c2f93..3f828fb456e2c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2898,9 +2898,9 @@ static int internal_get_user_pages_fast(unsigned long start,
 		might_lock_read(&current->mm->mmap_lock);
 
 	start = untagged_addr(start) & PAGE_MASK;
-	len = nr_pages << PAGE_SHIFT;
-	if (check_add_overflow(start, len, &end))
-		return 0;
+	if (check_shl_overflow(nr_pages, PAGE_SHIFT, &len) ||
+	    check_add_overflow(start, len, &end))
+		return -EOVERFLOW;
 	if (unlikely(!access_ok((void __user *)start, len)))
 		return -EFAULT;
 
-- 
2.53.0