linux-kernel - [PATCH] fbdev: Add bounds checking in bit

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Message-ID: <20250927075010.119671-1-albinbabuvarghese20@gmail.com>
Date: Sat, 27 Sep 2025 03:50:09 -0400
From: Albin Babu Varghese <albinbabuvarghese20@...il.com>
To: Simona Vetter <simona@...ll.ch>,
	Helge Deller <deller@....de>
Cc: Albin Babu Varghese <albinbabuvarghese20@...il.com>,
	syzbot+48b0652a95834717f190@...kaller.appspotmail.com,
	linux-fbdev@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

KASAN reports vmalloc-out-of-bounds writes in sys_imageblit during console
resize operations. The crash happens when bit_putcs renders characters 
outside the allocated framebuffer region.

Call trace: vc_do_resize -> clear_selection -> invert_screen ->
do_update_region -> fbcon_putcs -> bit_putcs -> sys_imageblit

The console resize changes dimensions but bit_putcs doesn't validate that 
the character positions fit within the framebuffer before rendering. 
This causes writes past the allocated buffer in fb_imageblit functions.

Fix by checking bounds before rendering:
- Return if dy + height > yres (would write past bottom)
- Break if dx + width > xres (would write past right edge)

Reported-by: syzbot+48b0652a95834717f190@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190  
Tested-by: syzbot+48b0652a95834717f190@...kaller.appspotmail.com
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@...il.com>
---
 drivers/video/fbdev/core/bitblit.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index f9475c14f733..4c732284384a 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -160,6 +160,9 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
 	image.height = vc->vc_font.height;
 	image.depth = 1;
 
+	if (image.dy + image.height > info->var.yres)
+		return;
+
 	if (attribute) {
 		buf = kmalloc(cellsize, GFP_ATOMIC);
 		if (!buf)
@@ -173,6 +176,10 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
 			cnt = count;
 
 		image.width = vc->vc_font.width * cnt;
+
+		if (image.dx + image.width > info->var.xres)
+			break;
+
 		pitch = DIV_ROUND_UP(image.width, 8) + scan_align;
 		pitch &= ~scan_align;
 		size = pitch * image.height + buf_align;
-- 
2.51.0