Re: [PATCH] macsec: avoid heap overflow in skb_to_sgvec

[Sabrina Dubroca <sd@queasysnail.net>]

2017-04-21, 23:14:48 +0200, Jason A. Donenfeld wrote:
> While this may appear as a humdrum one line change, it's actually quite
> important. An sk_buff stores data in three places:
> 
> 1. A linear chunk of allocated memory in skb->data. This is the easiest
>    one to work with, but it precludes using scatterdata since the memory
>    must be linear.
> 2. The array skb_shinfo(skb)->frags, which is of maximum length
>    MAX_SKB_FRAGS. This is nice for scattergather, since these fragments
>    can point to different pages.
> 3. skb_shinfo(skb)->frag_list, which is a pointer to another sk_buff,
>    which in turn can have data in either (1) or (2).
> 
> The first two are rather easy to deal with, since they're of a fixed
> maximum length, while the third one is not, since there can be
> potentially limitless chains of fragments. Fortunately dealing with
> frag_list is opt-in for drivers, so drivers don't actually have to deal
> with this mess. For whatever reason, macsec decided it wanted pain, and
> so it explicitly specified NETIF_F_FRAGLIST.
> 
> Because dealing with (1), (2), and (3) is insane, most users of sk_buff
> doing any sort of crypto or paging operation calls a convenient function
> called skb_to_sgvec (which happens to be recursive if (3) is in use!).
> This takes a sk_buff as input, and writes into its output pointer an
> array of scattergather list items. Sometimes people like to declare a
> fixed size scattergather list on the stack; othertimes people like to
> allocate a fixed size scattergather list on the heap. However, if you're
> doing it in a fixed-size fashion, you really shouldn't be using
> NETIF_F_FRAGLIST too (unless you're also ensuring the sk_buff and its
> frag_list children arent't shared and then you check the number of
> fragments in total required.)
> 
> Macsec specifically does this:
> 
>         size += sizeof(struct scatterlist) * (MAX_SKB_FRAGS + 1);
>         tmp = kmalloc(size, GFP_ATOMIC);
>         *sg = (struct scatterlist *)(tmp + sg_offset);
> 	...
>         sg_init_table(sg, MAX_SKB_FRAGS + 1);
>         skb_to_sgvec(skb, sg, 0, skb->len);
> 
> Specifying MAX_SKB_FRAGS + 1 is the right answer usually, but not if you're
> using NETIF_F_FRAGLIST, in which case the call to skb_to_sgvec will
> overflow the heap, and disaster ensues.

Ugh, good catch :/

AFAICT this patch doesn't really help, because NETIF_F_FRAGLIST
doesn't get tested in paths that can lead to triggering this.

I'll post a patch to allocate a properly-sized sg array.

-- 
Sabrina