netdev - Re: [PATCH bpf] bpf: fix precision tracking of stack slots

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Date:   Thu, 5 Sep 2019 15:15:00 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf] bpf: fix precision tracking of stack slots

On 9/4/19 12:16 AM, Alexei Starovoitov wrote:
> The problem can be seen in the following two tests:
> 0: (bf) r3 = r10
> 1: (55) if r3 != 0x7b goto pc+0
> 2: (7a) *(u64 *)(r3 -8) = 0
> 3: (79) r4 = *(u64 *)(r10 -8)
> ..
> 0: (85) call bpf_get_prandom_u32#7
> 1: (bf) r3 = r10
> 2: (55) if r3 != 0x7b goto pc+0
> 3: (7b) *(u64 *)(r3 -8) = r0
> 4: (79) r4 = *(u64 *)(r10 -8)
> 
> When backtracking need to mark R4 it will mark slot fp-8.
> But ST or STX into fp-8 could belong to the same block of instructions.
> When backtracing is done the parent state may have fp-8 slot
> as "unallocated stack". Which will cause verifier to warn
> and incorrectly reject such programs.
> 
> Writes into stack via non-R10 register are rare. llvm always
> generates canonical stack spill/fill.
> For such pathological case fall back to conservative precision
> tracking instead of rejecting.
> 
> Reported-by: syzbot+c8d66267fd2b5955287e@...kaller.appspotmail.com
> Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking")
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>

Applied, thanks!