Re: [PATCH] scsi: fix shift out-of-bounds in sg_build_indirect The num variable is set to 0. The variable num gets its value from scatter_elem_sz. However the minimum value of scatter_elem_sz is PAGE_SHIFT. So setting num to PAGE_SIZE when num < PAGE_SIZE.

[Khalid Aziz <khalid@kernel.org>]

On 10/6/25 11:46 AM, Kshitij Paranjape wrote:
> Cc: <stable@vger.kernel.org>
> Reported-by: syzbot+270f1c719ee7baab9941@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=270f1c719ee7baab9941
> Signed-off-by: Kshitij Paranjape <kshitijvparanjape@gmail.com>
> ---
>   drivers/scsi/sg.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index effb7e768165..9ae41bb256d7 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -1888,6 +1888,7 @@ sg_build_indirect(Sg_scatter_hold * schp, Sg_fd * sfp, int buff_size)
>   		if (num < PAGE_SIZE) {
>   			scatter_elem_sz = PAGE_SIZE;
>   			scatter_elem_sz_prev = PAGE_SIZE;
> +			num = scatter_elem_sz;
>   		} else
>   			scatter_elem_sz_prev = num;
>   	}

Have you seen any issues caused by not setting num to PAGE_SIZE when num 
< PAGE_SIZE?

 From what I can see, num is used to calculate the page order for 
allocation which will be 0 whether num=PAGE_SIZE or < PAGE_SIZE. After 
that num gets assigned a new value any way before its next use.

--
Khalid