netdev - Re: [PATCH bpf] bpf: fix ldx in ld_abs rewrite for large offsets

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Date:   Tue, 10 Jul 2018 13:27:31 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: fix ldx in ld_abs rewrite for large offsets

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 <syzkaller@...glegroups.com>
> >> Reported-by: Mark Rutland <mark.rutland@....com>
> >> Signed-off-by: Daniel Borkmann <daniel@...earbox.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.