linux-kernel - Re: [PATCH v2] fbdev: avoid out-of-bounds read in fb_pad_unaligned

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Message-ID: <889fd11b-80ea-4c23-b47f-4e6b17536b0f@gmx.de>
Date: Tue, 27 Jan 2026 18:57:32 +0100
From: Helge Deller <deller@....de>
To: Osama Abdelkader <osama.abdelkader@...il.com>,
 Simona Vetter <simona@...ll.ch>, Thomas Zimmermann <tzimmermann@...e.de>,
 Lee Jones <lee@...nel.org>, "Daniel Thompson (RISCstar)"
 <danielt@...nel.org>, Murad Masimov <m.masimov@...integration.ru>,
 Quanmin Yan <yanquanmin1@...wei.com>,
 Yongzhen Zhang <zhangyongzhen@...inos.cn>, linux-fbdev@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc: syzbot+55e03490a0175b8dd81d@...kaller.appspotmail.com
Subject: Re: [PATCH v2] fbdev: avoid out-of-bounds read in
 fb_pad_unaligned_buffer()

On 1/24/26 17:46, Osama Abdelkader wrote:
> fb_pad_unaligned_buffer() unconditionally reads and advances the source
> pointer for the final byte of each row, even when no bits from that byte
> are actually consumed.
> 
> When shift_high >= mod, the remaining bits do not cross a byte boundary,
> but the code still accesses the next source byte. This can lead to
> out-of-bounds reads under malformed geometry, as reported by syzbot.
> 
> Fix this by only accessing and consuming the final source byte when it
> contributes bits (shift_high < mod).
> 
> This fixes the KASAN slab-out-of-bounds read reported by syzkaller:
> https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d
> 
> Reported-by: syzbot+55e03490a0175b8dd81d@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d
> Signed-off-by: Osama Abdelkader <osama.abdelkader@...il.com>
> ---
> v2: address the real issue (shift_high >= mod) condition.
> ---
>   drivers/video/fbdev/core/fbmem.c | 15 +++++++++------
>   1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index eff757ebbed1..d125c3db37a1 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -100,7 +100,7 @@ EXPORT_SYMBOL(fb_pad_aligned_buffer);
>   void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height,
>   				u32 shift_high, u32 shift_low, u32 mod)
>   {
> -	u8 mask = (u8) (0xfff << shift_high), tmp;
> +	u8 mask = (u8) (0xff << shift_high), tmp;

This part is correct, but shouldn't be part of this patch.
  


>   	int i, j;
>   
>   	for (i = height; i--; ) {
> @@ -113,15 +113,18 @@ void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height,
>   			dst[j+1] = tmp;
>   			src++;
>   		}
> -		tmp = dst[idx];
> -		tmp &= mask;
> -		tmp |= *src >> shift_low;
> -		dst[idx] = tmp;
> +
> +		/* Only consume another source byte if it contributes bits */
>   		if (shift_high < mod) {
> +			tmp = dst[idx];
> +			tmp &= mask;
> +			tmp |= *src >> shift_low;
> +			dst[idx] = tmp;
>   			tmp = *src << shift_high;
>   			dst[idx+1] = tmp;
> +			src++;
>   		}
> -		src++;

Above you moved the src pointer inside the if(), so every line
processed may miss a ptr increment. This means the source would need to
be different too, but it hasn't changed, as it's still used from
bit_putcs_unaligned() which prints a char from the character fonts.

So, I believe this part at least is wrong.
Did you test it?

Helge