Re: [PATCH] net/hamradio/6pack: Fix the size of a sk_buff used in 'sp_bump()'

[David Miller]

From: Christophe JAILLET 
Date: Mon, 26 Aug 2019 21:02:09 +0200

> We 'allocate' 'count' bytes here. In fact, 'dev_alloc_skb' already add some
> extra space for padding, so a bit more is allocated.
> 
> However, we use 1 byte for the KISS command, then copy 'count' bytes, so
> count+1 bytes.
> 
> Explicitly allocate and use 1 more byte to be safe.
> 
> Signed-off-by: Christophe JAILLET 

I applied your patch as-is, as it is correct and doesn't change the contents
of the data put into the SKB at all.

->rcount is the cooked count minus two, but then we copy effectively
cooked count minus one bytes from one byte past the beginning of the
cooked buffer and so all the accesses are in range on the input buffer
side.

Re: [PATCH] net/hamradio/6pack: Fix the size of a sk_buff used in 'sp_bump()'

[David Miller]

From: Christophe JAILLET 
Date: Mon, 26 Aug 2019 21:02:09 +0200

> We 'allocate' 'count' bytes here. In fact, 'dev_alloc_skb' already add some
> extra space for padding, so a bit more is allocated.
> 
> However, we use 1 byte for the KISS command, then copy 'count' bytes, so
> count+1 bytes.
> 
> Explicitly allocate and use 1 more byte to be safe.
> 
> Signed-off-by: Christophe JAILLET 
> ---
> This patch should be safe, be however may no be the correct way to fix the
> "buffer overflow". Maybe, the allocated size is correct and we should have:
>memcpy(ptr, sp->cooked_buf + 1, count - 1);
> or
>memcpy(ptr, sp->cooked_buf + 1, count - 1sp->rcount);
> 
> I've not dig deep enough to understand the link betwwen 'rcount' and
> how 'cooked_buf' is used.

I'm trying to figure out how this code works too.

Why are they skipping over the first byte?  Is that to avoid the
command byte?  Yes, then using sp->rcount as the memcpy length makes
sense.

Why is the caller subtracting 2 from the RX buffer count when
calculating sp->rcount?  This makes the situation even more confusing.