linux-kernel - Re: stm class: Prevent user-controllable allocations

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Message-ID: <20190507125027.GV2239@kadam>
Date:   Tue, 7 May 2019 15:50:27 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     alexander.shishkin@...ux.intel.com, security@...nel.org,
        sasha.levin@...cle.com, gregkh@...uxfoundation.org,
        kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...l.org>, trivial@...nel.org
Subject: Re: stm class: Prevent user-controllable allocations

On Tue, May 07, 2019 at 02:41:13PM +0200, Pavel Machek wrote:
> 
> It seems to me that we still allow overflow if count == ~0. We'll then
> allocate 0 bytes but copy ~0 bytes. That does not sound healthy.
> 
> Fixes: f08b18266c7116e2ec6885dd53a928f580060a71
> 
> Signed-off-by: Pavel Machek <pavel@...x.de>
> 
> diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
> index c7ba8ac..8846fca 100644
> --- a/drivers/hwtracing/stm/core.c
> +++ b/drivers/hwtracing/stm/core.c
> @@ -631,7 +631,7 @@ static ssize_t stm_char_write(struct file *file, const char __user *buf,
>  	char *kbuf;
>  	int err;
>  
> -	if (count + 1 > PAGE_SIZE)
> +	if (count > PAGE_SIZE - 1)
>  		count = PAGE_SIZE - 1;

The "count" variable should all be checked in vfs_write().  count + off
is checked in rw_verify_area() and count is capped at MAX_RW_COUNT.

#define MAX_RW_COUNT (INT_MAX & PAGE_MASK)

regards,
dan carpenter