linux-kernel - [PATCH][next] io_uring/kbuf: fix unintentional sign extension on shift of reg.bgid

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Message-Id: <20241204153923.401674-1-colin.i.king@gmail.com>
Date: Wed,  4 Dec 2024 15:39:23 +0000
From: Colin Ian King <colin.i.king@...il.com>
To: Jens Axboe <axboe@...nel.dk>,
	Pavel Begunkov <asml.silence@...il.com>,
	io-uring@...r.kernel.org
Cc: kernel-janitors@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH][next] io_uring/kbuf: fix unintentional sign extension on shift of reg.bgid

Shifting reg.bgid << IORING_OFF_PBUF_SHIFT results in a promotion
from __u16 to a 32 bit signed integer, this is then sign extended
to a 64 bit unsigned long on 64 bit architectures. If reg.bgid is
greater than 0x7fff then this leads to a sign extended result where
all the upper 32 bits of mmap_offset are set to 1. Fix this by
casting reg.bgid to the same type as mmap_offset before performing
the shift.

Fixes: ff4afde8a61f ("io_uring/kbuf: use region api for pbuf rings")
Signed-off-by: Colin Ian King <colin.i.king@...il.com>
---
 io_uring/kbuf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index e91260a6156b..15e5e6ec5968 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -640,7 +640,7 @@ int io_register_pbuf_ring(struct io_ring_ctx *ctx, void __user *arg)
 			return -ENOMEM;
 	}
 
-	mmap_offset = reg.bgid << IORING_OFF_PBUF_SHIFT;
+	mmap_offset = (unsigned long)reg.bgid << IORING_OFF_PBUF_SHIFT;
 	ring_size = flex_array_size(br, bufs, reg.ring_entries);
 
 	memset(&rd, 0, sizeof(rd));
-- 
2.39.5