Commit 3637d34b authored Oct 03, 2025 by Albin Babu Varghese Committed by Helge Deller Oct 04, 2025

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@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190

Suggested-by: Helge Deller <deller@gmx.de>
Tested-by: <syzbot+48b0652a95834717f190@syzkaller.appspotmail.com>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>

parent c8fee6a7

drivers/video/fbdev/core/bitblit.c

+17 −0

Original line number	Diff line number	Diff line
		@@ -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 +178,18 @@ 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;