Thanks for pointing this out. It seems that we would need to allocate
this extra byte somewhere in kernel ppp code.
However, I am not very familiar with that part of the code yet, and
I'll appreciate any help with finding parts of code where obuffer for
mppe_decompress is allocated.
On 6/20/07, Sergey Vlasov <[EMAIL PROTECTED]> wrote:
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 <[EMAIL PROTECTED]>
> ---
> --- 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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