linux-kernel - Re: Bug report - slab-out-of-bounds in vcs_scr

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Date:   Tue, 5 Nov 2019 07:54:21 +0100
From:   Jiri Slaby <jslaby@...e.com>
To:     Nicolas Pitre <nico@...xnic.net>,
        Or Cohen <orcohen@...oaltonetworks.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, textshell@...uujin.de,
        Daniel Vetter <daniel.vetter@...ll.ch>, sam@...nborg.org,
        mpatocka@...hat.com, ghalat@...hat.com,
        linux-kernel@...r.kernel.org, jwilk@...lk.net,
        Nadav Markus <nmarkus@...oaltonetworks.com>,
        syzkaller@...glegroups.com
Subject: Re: Bug report - slab-out-of-bounds in vcs_scr_readw

On 04. 11. 19, 19:33, Nicolas Pitre wrote:
> On Mon, 4 Nov 2019, Or Cohen wrote:
> 
>> @gregkh@...uxfoundation.org @nico@...xnic.net  - Thanks for the quick response.
>> @gregkh@...uxfoundation.org  - 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@...libre.com>
> 
> 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