On Thu, Jun 29, 2017 at 03:04:59AM +0200, Daniel Borkmann wrote:
> Leaking kernel addresses on unpriviledged is generally disallowed,
> for example, verifier rejects the following:
>
> 0: (b7) r0 = 0
> 1: (18) r2 = 0xffff897e82304400
> 3: (7b) *(u64 *)(r1 +48) = r2
> R2 leaks addr into ctx
>
> Doing pointer arithmetic on them is also forbidden, so that they
> don't turn into unknown value and then get leaked out. However,
> there's xadd as a special case, where we don't check the src reg
> for being a pointer register, e.g. the following will pass:
>
> 0: (b7) r0 = 0
> 1: (7b) *(u64 *)(r1 +48) = r0
> 2: (18) r2 = 0xffff897e82304400 ; map
> 4: (db) lock *(u64 *)(r1 +48) += r2
> 5: (95) exit
>
> We could store the pointer into skb->cb, loose the type context,
> and then read it out from there again to leak it eventually out
> of a map value. Or more easily in a different variant, too:
>
> 0: (bf) r6 = r1
> 1: (7a) *(u64 *)(r10 -8) = 0
> 2: (bf) r2 = r10
> 3: (07) r2 += -8
> 4: (18) r1 = 0x0
> 6: (85) call bpf_map_lookup_elem#1
> 7: (15) if r0 == 0x0 goto pc+3
> R0=map_value(ks=8,vs=8,id=0),min_value=0,max_value=0 R6=ctx R10=fp
> 8: (b7) r3 = 0
> 9: (7b) *(u64 *)(r0 +0) = r3
> 10: (db) lock *(u64 *)(r0 +0) += r6
> 11: (b7) r0 = 0
> 12: (95) exit
>
> from 7 to 11: R0=inv,min_value=0,max_value=0 R6=ctx R10=fp
> 11: (b7) r0 = 0
> 12: (95) exit
>
> Prevent this by checking xadd src reg for pointer types. Also
> add a couple of test cases related to this.
>
> Fixes: 1be7f75d1668 ("bpf: enable non-root eBPF programs")
> Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
> Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
Acked-by: Martin KaFai Lau <ka...@fb.com>
