[PATCH v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

[PATCH v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

[Albin Babu Varghese]

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>
---
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

Re: [PATCH v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

[Helge Deller]

On 10/3/25 09:32, Albin Babu Varghese wrote:
> 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>
> ---
> 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(+)

applied.

Thanks!
Helge

Re: [PATCH v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

[Albin Babu Varghese]

On Sat, Oct 04, 2025 at 02:43:33AM +0200, Helge Deller wrote:
> On 10/3/25 09:32, Albin Babu Varghese wrote:
> > 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>
> > ---
> > 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(+)
> 
> applied.
> 
> Thanks!
> Helge
Thank you for merging the patch.

After the patch appeared in mainline, I observed that syzbot continues
to find the same bug through different execution paths. My fix addressed
bit_putcs, but the crashes now occur through bit_cursor and cw_putcs,
which bypass bit_putcs entirely and go directly to sys_imageblit():

Crash 1 (cursor path):
https://syzkaller.appspot.com/text?tag=CrashReport&x=11fe95e2580000
Call trace: hide_cursor → bit_cursor → soft_cursor → sys_imageblit

Crash 2 (rotation path):
https://syzkaller.appspot.com/text?tag=CrashReport&x=164f0b04580000
Call trace: fbcon_modechanged → cw_putcs → sys_imageblit

The original syzbot reproducer depended on character height going out of
bounds, so I focused on bit_putcs where character images are drawn. I
naively overlooked cursor drawing - apologies for that. That's why I
looked at the other crash reports after the merge, because it seemed odd
that it was still hitting the bug after the fix.

I believe adding the same clipping logic in sys_imageblit() would provide
comprehensive protection. Something like this:

void sys_imageblit(struct fb_info *p, const struct fb_image *image)
  {
 +       struct fb_image clipped;
 +       u32 width, height;
 +
         if (!(p->flags & FBINFO_VIRTFB))
                 fb_warn_once(p, "%s: framebuffer is not in virtual address space.\n", __func__);
 
 -       fb_imageblit(p, image);
 +       if (!image || image->dx >= p->var.xres || image->dy >= p->var.yres)
 +               return;
 +
 +       if (image->dx + image->width > p->var.xres || image->dy + image->height > p->var.yres) {
 +               clipped = *image;
 +
 +               height = clipped.height;
 +               if (clipped.dy + height > p->var.yres)
 +                       height = p->var.yres - clipped.dy;
 +
 +               clipped.height = height;
 +
 +               width = clipped.width;
 +               if (clipped.dx + width > p->var.xres)
 +                       width = p->var.xres - clipped.dx;
 +
 +               clipped.width = width;
 +
 +               if (clipped.width == 0 || clipped.height == 0)
 +                       return;
 +
 +               fb_imageblit(p, &clipped);
 +       } else {
 +               fb_imageblit(p, image);
 +       }
  }
 
I don't have any way to test this other than testing it with the existing
reproducers. I can prepare a patch if this approach sounds reasonable.

Thanks,
Albin