Re: [PATCH 2.6.21.3] ppp_mppe: account for osize too small errors in mppe_decompress()

[Sergey Vlasov <vsu@altlinux.ru>]

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On Tue, 05 Jun 2007 22:23:02 +1100 Konstantin Sharlaimov wrote:

> Prevent mppe_decompress() from generating "osize too small" errors when checking
> for output buffer size. When receiving a packet of mru size the output buffer
> for decrypted data is 1 byte too small since mppe_decompress() tries to account
> for possible PFC, however later in code it is assumed no PFC.
>
> Adjusting the check prevented there errors from occurring.
>
> Signed-off-by: Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
> ---
> --- linux-2.6.21.3/drivers/net/ppp_mppe.c.orig	2007-06-01 20:57:04.000000000 +1100
> +++ linux-2.6.21.3/drivers/net/ppp_mppe.c	2007-06-05 21:46:30.000000000 +1100
> @@ -493,14 +493,14 @@ mppe_decompress(void *arg, unsigned char
>
>  	/*
>  	 * Make sure we have enough room to decrypt the packet.
> -	 * Note that for our test we only subtract 1 byte whereas in
> -	 * mppe_compress() we added 2 bytes (+MPPE_OVHD);
> -	 * this is to account for possible PFC.
> +	 * To account for possible PFC we should only subtract 1
> +	 * byte whereas in mppe_compress() we added 2 bytes (+MPPE_OVHD);
> +	 * However, we assume no PFC, thus subtracting 2 bytes.
>  	 */
> -	if (osize < isize - MPPE_OVHD - 1) {
> +	if (osize < isize - MPPE_OVHD - 2) {
>  		printk(KERN_DEBUG "mppe_decompress[%d]: osize too small! "
>  		       "(have: %d need: %d)\n", state->unit,
> -		       osize, isize - MPPE_OVHD - 1);
> +		       osize, isize - MPPE_OVHD - 2);
>  		return DECOMP_ERROR;
>  	}
>  	osize = isize - MPPE_OVHD - 2;	/* assume no PFC */

Seems that this patch is not correct - even though the comment above
says "assume no PFC", the subsequent code actually supports PFC:

	/*
	 * Do PFC decompression.
	 * This would be nicer if we were given the actual sk_buff
	 * instead of a char *.
	 */
	if ((obuf[0] & 0x01) != 0) {
		obuf[1] = obuf[0];
		obuf[0] = 0;
		obuf++;
		osize++;
	}

Therefore, depending on the packet contents, the decompressor may
really need that extra byte in the output buffer, and changing the
check may cause buffer overflows.

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