Re: Bug report - slab-out-of-bounds in vcs_scr_readw

[Jiri Slaby]

On 04. 11. 19, 19:33, Nicolas Pitre wrote:
> On Mon, 4 Nov 2019, Or Cohen wrote:
> 
>> @gregkh@xxxxxxxxxxxxxxxxxxx @nico@xxxxxxxxxxx  - Thanks for the quick response.
>> @gregkh@xxxxxxxxxxxxxxxxxxx  - Regarding your question, I don't think
>> the 1 byte buffer is related to the problem. (  it's just was there in
>> the initial reproducer the fuzzer created, and I forgot to remove it
>> while reducing code from the reproducer ).
> 
> I think I know what the problem is. I have no time to test it though.
> 
> Please try this (untested) patch. Also please try running the same test 
> code but with vcsa6 in addition to vcsu6 to be sure.
> 
> ---------- >8
> Subject: [PATCH] vcs: add missing validation on vcs_size() returned value
> 
> One usage instance didn't account for the fact that vcs_size() may
> return a negative error code.
> 
> Signed-off-by: Nicolas Pitre <npitre@xxxxxxxxxxxx>
> 
> diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
> index 1f042346e7..fa07d79027 100644
> --- a/drivers/tty/vt/vc_screen.c
> +++ b/drivers/tty/vt/vc_screen.c
> @@ -474,6 +474,10 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
>  		goto unlock_out;
>  
>  	size = vcs_size(inode);
> +	if (size < 0) {
> +		ret = size;
> +		goto unlock_out;
> +	}
>  	ret = -EINVAL;
>  	if (pos < 0 || pos > size)
>  		goto unlock_out;

pos must be >= 0, so "pos > size" would catch this case as a side
effect, or am I missing something? That being said, the patch is
correct, but won't fix the issue IMO.

thanks,
-- 
js
suse labs