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From:   Yangxi Xiang <xyangxi5@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Johan Hovold <johan@kernel.org>,
        Igor Matheus Andrade Torrente <igormtorrente@gmail.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        nick black <dankamongmen@gmail.com>,
        Yangxi Xiang <xyangxi5@gmail.com>
Subject: [PATCH v2] vt: fix memory overlapping when deleting chars in the buffer
Date:   Mon, 27 Jun 2022 20:54:28 +0800
Message-Id: <20220627125428.19885-1-xyangxi5@gmail.com>
Precedence: bulk

A memory overlapping copy occurs when deleting a long line. This memory
overlapping copy can cause data corruption when scr_memcpyw is optimized
to memcpy because memcpy does not ensure its behavior if the destination
buffer overlaps with the source buffer. The line buffer is not always
broken, because the memcpy utilizes the hardware acceleration, whose
result is not deterministic.

Fix this problem by using replacing the scr_memcpyw with scr_memmovew, and
preserving the memcpy optimization when the buffers are not overlapping.

Fixes: 81732c3b2fed ("Fix line garbage in virtual console").
Signed-off-by: Yangxi Xiang <xyangxi5@gmail.com>
---
 drivers/tty/vt/vt.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index f8c87c4d7399..d87bff9d8ed5 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -853,9 +853,13 @@ static void insert_char(struct vc_data *vc, unsigned int nr)
 static void delete_char(struct vc_data *vc, unsigned int nr)
 {
 	unsigned short *p = (unsigned short *) vc->vc_pos;
+	unsigned short cp = (vc->vc_cols - vc->state.x - nr) * 2;
 
 	vc_uniscr_delete(vc, nr);
-	scr_memcpyw(p, p + nr, (vc->vc_cols - vc->state.x - nr) * 2);
+	if (cp > nr)
+		scr_memmovew(p, p + nr, cp);
+	else
+		scr_memcpyw(p, p + nr, cp);
 	scr_memsetw(p + vc->vc_cols - vc->state.x - nr, vc->vc_video_erase_char,
 			nr * 2);
 	vc->vc_need_wrap = 0;
-- 
2.17.1