netdev - Re: [PATCH net] bpf: fix states equal logic for varlen access

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Message-ID: <20161129030446.GA13735@ast-mbp.thefacebook.com>
Date:   Mon, 28 Nov 2016 19:04:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josef Bacik <jbacik@...com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, daniel@...earbox.net,
        ast@...nel.org, jannh@...gle.com
Subject: Re: [PATCH net] bpf: fix states equal logic for varlen access

On Mon, Nov 28, 2016 at 02:44:10PM -0500, Josef Bacik wrote:
> If we have a branch that looks something like this
> 
> int foo = map->value;
> if (condition) {
>   foo += blah;
> } else {
>   foo = bar;
> }
> map->array[foo] = baz;
> 
> We will incorrectly assume that the !condition branch is equal to the condition
> branch as the register for foo will be UNKNOWN_VALUE in both cases.  We need to
> adjust this logic to only do this if we didn't do a varlen access after we
> processed the !condition branch, otherwise we have different ranges and need to
> check the other branch as well.
> 
> Signed-off-by: Josef Bacik <jbacik@...com>
> ---
>  kernel/bpf/verifier.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 89f787c..2c8a688 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2478,6 +2478,7 @@ static bool states_equal(struct bpf_verifier_env *env,
>  {
>  	struct bpf_reg_state *rold, *rcur;
>  	int i;
> +	bool map_access = env->varlen_map_value_access;

that's a bit misleading name for the variable.
Pls call it varlen_map_access.

>  	for (i = 0; i < MAX_BPF_REG; i++) {
>  		rold = &old->regs[i];
> @@ -2489,12 +2490,17 @@ static bool states_equal(struct bpf_verifier_env *env,
>  		/* If the ranges were not the same, but everything else was and
>  		 * we didn't do a variable access into a map then we are a-ok.
>  		 */
> -		if (!env->varlen_map_value_access &&
> +		if (!map_access &&
>  		    rold->type == rcur->type && rold->imm == rcur->imm)

just noticed that this one is missing comparing rold->id == rcur->id

>  			continue;
>  
> +		/* If we didn't map access then again we don't care about the
> +		 * mismatched range values and it's ok if our old type was
> +		 * UNKNOWN and we didn't go to a NOT_INIT'ed reg.
> +		 */
>  		if (rold->type == NOT_INIT ||
> -		    (rold->type == UNKNOWN_VALUE && rcur->type != NOT_INIT))
> +		    (!map_access && (rold->type == UNKNOWN_VALUE &&
> +				     rcur->type != NOT_INIT)))

please drop unnecessary ( )