linux-kernel - Re: [PATCH] sound/oss/midi_synth: prevent underflow, use of uninitialized value, and signedness issue

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Date:	Wed, 23 Mar 2011 08:39:22 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	perex@...ex.cz, alsa-devel@...a-project.org, security@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sound/oss/midi_synth: prevent underflow, use of uninitialized value, and signedness issue

At Tue, 22 Mar 2011 14:31:08 -0400,
Dan Rosenberg wrote:
> 
> The offset passed to midi_synth_load_patch() can be essentially
> arbitrary.  If it's greater than the header length, this will result in
> a copy_from_user(dst, src, negative_val).  While this will just return
> -EFAULT on x86, on other architectures this may cause memory corruption.
> Additionally, the length field of the sysex_info structure may not be
> initialized prior to its use.  Finally, a signed comparison may result
> in an unintentionally large loop.
> 
> This patch fixes all these issues, as well as some cleanup to prevent
> checkpatch.pl from complaining.
> 
> Signed-off-by: Dan Rosenberg <drosenberg@...curity.com>
> Cc: stable@...nel.org

Well, the whole load_patch mechanism doesn't look working right with
ofs != 0.  The only caller of this callback is sound/oss/sequencer.c,
and it assumes that the whole chunk is passed once without splitting.
Thus the offset calculation in this code is obviously wrong, and
passing offset itself doesn't make any sense.

A similar problem (uninitialized struct fields) is found in another
load_patch callback in sound/oss/opl3.c.

That is, the best fix would be to rip off the offset argument from
this callback.


thanks,

Takashi

> ---
>  sound/oss/midi_synth.c |   27 ++++++++++++++-------------
>  1 files changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/sound/oss/midi_synth.c b/sound/oss/midi_synth.c
> index 3c09374..3500f80 100644
> --- a/sound/oss/midi_synth.c
> +++ b/sound/oss/midi_synth.c
> @@ -491,16 +491,18 @@ midi_synth_load_patch(int dev, int format, const char __user *addr,
>  	if (!prefix_cmd(orig_dev, 0xf0))
>  		return 0;
>  
> +	/* Invalid patch format */
>  	if (format != SYSEX_PATCH)
> -	{
> -/*		  printk("MIDI Error: Invalid patch format (key) 0x%x\n", format);*/
>  		  return -EINVAL;
> -	}
> +
> +	/* Patch header too short */
>  	if (count < hdr_size)
> -	{
> -/*		printk("MIDI Error: Patch header too short\n");*/
>  		return -EINVAL;
> -	}
> +
> +	/* Offset too high */
> +	if (offs > offsetof(struct sysex_info, len) || offs < 0)
> +		return -EINVAL;
> +
>  	count -= hdr_size;
>  
>  	/*
> @@ -510,14 +512,13 @@ midi_synth_load_patch(int dev, int format, const char __user *addr,
>  
>  	if(copy_from_user(&((char *) &sysex)[offs], &(addr)[offs], hdr_size - offs))
>  		return -EFAULT;
> - 
> - 	if (count < sysex.len)
> -	{
> -/*		printk(KERN_WARNING "MIDI Warning: Sysex record too short (%d<%d)\n", count, (int) sysex.len);*/
> +
> +	/* Sysex record too short */
> +	if ((unsigned)count < (unsigned)sysex.len)
>  		sysex.len = count;
> -	}
> -  	left = sysex.len;
> -  	src_offs = 0;
> +
> +	left = sysex.len;
> +	src_offs = 0;
>  
>  	for (i = 0; i < left && !signal_pending(current); i++)
>  	{
> 
> 
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