linux-kernel - [PATCH v2] fbdev: Add bounds checking in bit

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Message-ID: <20251003073210.48501-1-albinbabuvarghese20@gmail.com>
Date: Fri,  3 Oct 2025 03:32:09 -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 v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

Add bounds checking to prevent writes past framebuffer boundaries when
rendering text near screen edges. Return early if the Y position is off-screen
and clip image height to screen boundary. Break from the rendering loop if the
X position is off-screen. When clipping image width to fit the screen, update
the character count to match the clipped width to prevent buffer size
mismatches.

Without the character count update, bit_putcs_aligned and bit_putcs_unaligned
receive mismatched parameters where the buffer is allocated for the clipped
width but cnt reflects the original larger count, causing out-of-bounds writes.

Reported-by: syzbot+48b0652a95834717f190@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190  
Suggested-by: Helge Deller <deller@....de>
Tested-by: syzbot+48b0652a95834717f190@...kaller.appspotmail.com
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@...il.com>
---
Changes in v2:
- Partially render when height exceeding screen boundaries instead of skipping 
- Update character count when width is clipped to prevent buffer mismatch

Link to v1: 
https://lore.kernel.org/all/20250927075010.119671-1-albinbabuvarghese20@gmail.com/
---
 drivers/video/fbdev/core/bitblit.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index f9475c14f733..116b4bf5e94c 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -160,6 +160,11 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
 	image.height = vc->vc_font.height;
 	image.depth = 1;
 
+	if (image.dy >= info->var.yres)
+		return;
+
+	image.height = min(image.height, info->var.yres - image.dy);
+
 	if (attribute) {
 		buf = kmalloc(cellsize, GFP_ATOMIC);
 		if (!buf)
@@ -173,6 +178,21 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
 			cnt = count;
 
 		image.width = vc->vc_font.width * cnt;
+
+		if (image.dx >= info->var.xres)
+			break;
+
+		if (image.dx + image.width > info->var.xres) {
+			image.width = info->var.xres - image.dx;
+
+			cnt = image.width / vc->vc_font.width;
+
+			if (cnt == 0)
+				break;
+
+			image.width = cnt * vc->vc_font.width;
+		}
+
 		pitch = DIV_ROUND_UP(image.width, 8) + scan_align;
 		pitch &= ~scan_align;
 		size = pitch * image.height + buf_align;
-- 
2.51.0