On Tue, Jul 10, 2018 at 02:13:42PM +0200, Daniel Borkmann wrote:
> On 07/10/2018 12:14 PM, Mark Rutland wrote:
> > On Tue, Jul 10, 2018 at 12:43:22AM +0200, Daniel Borkmann wrote:
> >> Mark reported that syzkaller triggered a KASAN detected slab-out-of-bounds
> >> bug in ___bpf_prog_run() with a BPF_LD | BPF_ABS word load at offset
> >> 0x8001.
> >> After further investigation it became clear that the issue was the
> >> BPF_LDX_MEM() which takes offset as an argument whereas it cannot encode
> >> larger than S16_MAX offsets into it. For this synthetical case we need to
> >> move the full address into tmp register instead and do the LDX without
> >> immediate value.
> >>
> >> Fixes: e0cea7ce988c ("bpf: implement ld_abs/ld_ind in native bpf")
> >> Reported-by: syzbot <syzkal...@googlegroups.com>
> >> Reported-by: Mark Rutland <mark.rutl...@arm.com>
> >> Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
> >> ---
> >> net/core/filter.c | 16 +++++++++++++---
> >> 1 file changed, 13 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/net/core/filter.c b/net/core/filter.c
> >> index 5fa66a3..a13f5b1 100644
> >> --- a/net/core/filter.c
> >> +++ b/net/core/filter.c
> >> @@ -459,11 +459,21 @@ static bool convert_bpf_ld_abs(struct sock_filter
> >> *fp, struct bpf_insn **insnp)
> >> (!unaligned_ok && offset >= 0 &&
> >> offset + ip_align >= 0 &&
> >> offset + ip_align % size == 0))) {
> >> + bool ldx_off_ok = offset <= S16_MAX;
> >> +
> >
> > Given offset is a (signed) int, is it possible for that to be a negative
> > value less than S16_MIN? ... or is that ruled out elsewhere?
>
> This branch here handles only positive offset.
Ah, sorry. I missed the "offset >= 0" check in the context above.
> offset can be negative,
> but in that case these insns won't be emitted and handling will be done
> in 'slow path' via bpf_skb_load_helper_*().
Ok; looks good to me, then.
Thanks,
Mark.
