netdev - Re: [PATCH bpf] bpf: verifier: make sure callees don't prune with caller differences

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Message-ID: <64af6c24-d21a-d149-af48-3b7eaf6cdb25@solarflare.com>
Date:   Wed, 12 Dec 2018 20:28:07 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        <alexei.starovoitov@...il.com>, <daniel@...earbox.net>
CC:     <oss-drivers@...ronome.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: verifier: make sure callees don't prune with
 caller differences

On 10/12/18 19:35, Jakub Kicinski wrote:
> Currently for liveness and state pruning the register parentage
> chains don't include states of the callee.  This makes some sense
> as the callee can't access those registers.  However, this means
> that READs done after the callee returns will not propagate into
> the states of the callee.  Callee will then perform pruning
> disregarding differences in caller state.
>
> Example:
>
>    0: (85) call bpf_user_rnd_u32
>    1: (b7) r8 = 0
>    2: (55) if r0 != 0x0 goto pc+1
>    3: (b7) r8 = 1
>    4: (bf) r1 = r8
>    5: (85) call pc+4
>    6: (15) if r8 == 0x1 goto pc+1
>    7: (05) *(u64 *)(r9 - 8) = r3
>    8: (b7) r0 = 0
>    9: (95) exit
>
>    10: (15) if r1 == 0x0 goto pc+0
>    11: (95) exit
>
> Here we acquire unknown state with call to get_random() [1].  Then
> we store this random state in r8 (either 0 or 1) [1 - 3], and make
> a call on line 5.  Callee does nothing but a trivial conditional
> jump (to create a pruning point).  Upon return caller checks the
> state of r8 and either performs an unsafe read or not.
>
> Verifier will first explore the path with r8 == 1, creating a pruning
> point at [11].  The parentage chain for r8 will include only callers
> states so once verifier reaches [6] it will mark liveness only on states
> in the caller, and not [11].  Now when verifier walks the paths with
> r8 == 0 it will reach [11] and since REG_LIVE_READ on r8 was not
> propagated there it will prune the walk entirely (stop walking
> the entire program, not just the callee).  Since [6] was never walked
> with r8 == 0, [7] will be considered dead and replaced with "goto -1"
> causing hang at runtime.
>
> This patch weaves the callee's explored states onto the callers
> parentage chain.
Could you describe this in a little more detail?

In particular I'm not sure why anything that knows the difference between
 caller- and callee-saved regs belongs in is_state_visited() rather than
 check_func_call().  Is that just an optimisation that we can do because
 we know caller's r1-r5 were scratched by the call?

It looks like, without that, the change is "every reg in all frames needs
 to be parented to the new-state", which is somewhat easier to understand
 (though I did have to think for quite a long time before it made sense
 to me why even that was necessary ;-)

So please add more to the patch description, and maybe a comment above
 the loop explaining why it's r6-r9 only; then I'll be happy to Ack.

> v1: don't unnecessarily link caller saved regs (Jiong)
>
> Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
> Reported-by: David Beckett <david.beckett@...ronome.com>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Jiong Wang <jiong.wang@...ronome.com>