lkml.org 
[lkml]   [2017]   [Apr]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH] macsec: avoid heap overflow in skb_to_sgvec
Date
From: Jason A. Donenfeld
> Sent: 21 April 2017 22:15
> 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.
...

Shouldn't skb_to_sgvec() be checking the number of fragments against
the size of the sg list?
The callers would then all need auditing to allow for failure.

David


\
 
 \ /
  Last update: 2017-04-24 13:07    [W:0.063 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site