FAILED: patch "[PATCH] mm/gup: check for overflow when converting nr_pages to bytes in GUP-fast" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 21a3888d3d2f87b7f15ccea1f0befd57346fe899
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109202323-failed-5.15-21a3888d3d2f@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 21a3888d3d2f87b7f15ccea1f0befd57346fe899 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: Sat, 31 Oct 2026 10:39:51 +0100
Subject: [PATCH] mm/gup: check for overflow when converting nr_pages to bytes
 in GUP-fast

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>
---
 mm/gup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 1c2ac7c66c309..ba4907b352323 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3225,8 +3225,8 @@ static int gup_fast_fallback(unsigned long start, unsigned long nr_pages,
 		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))
+	if (check_shl_overflow(nr_pages, PAGE_SHIFT, &len) ||
+	    check_add_overflow(start, len, &end))
 		return -EOVERFLOW;
 	if (end > TASK_SIZE_MAX)
 		return -EFAULT;
-- 
2.53.0