netdev - Re: [PATCH bpf-next 1/5] bpf: Reject indirect var

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Message-ID: <20190403215754.GB80278@rdna-mbp.dhcp.thefacebook.com>
Date:   Wed, 3 Apr 2019 21:57:56 +0000
From:   Andrey Ignatov <rdna@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/5] bpf: Reject indirect var_off stack access in
 raw mode

Daniel Borkmann <daniel@...earbox.net> [Wed, 2019-04-03 09:46 -0700]:
> On 04/03/2019 06:21 PM, Daniel Borkmann wrote:
> > On 04/02/2019 10:19 PM, Andrey Ignatov wrote:
> >> It's hard to guarantee that whole memory is marked as initialized on
> >> helper return if uninitialized stack is accessed with variable offset
> >> since specific bounds are unknown to verifier. This may cause
> >> uninitialized stack leaking.
> >>
> >> Reject such an access in check_stack_boundary to prevent possible
> >> leaking.
> >>
> >> There are no known use-cases for indirect uninitialized stack access
> >> with variable offset so it shouldn't break anything.
> >>
> >> Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers")
> >> Reported-by: Daniel Borkmann <daniel@...earbox.net>
> >> Signed-off-by: Andrey Ignatov <rdna@...com>
> >> ---
> >>  kernel/bpf/verifier.c | 25 +++++++++++++++++++------
> >>  1 file changed, 19 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >> index b7a7a9caa82f..12b84307ffa8 100644
> >> --- a/kernel/bpf/verifier.c
> >> +++ b/kernel/bpf/verifier.c
> >> @@ -2212,7 +2212,26 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
> >>  					     zero_size_allowed);
> >>  		if (err)
> >>  			return err;
> >> +		if (meta && meta->raw_mode) {
> >> +			meta->access_size = access_size;
> >> +			meta->regno = regno;
> >> +			return 0;
> >> +		}
> >>  	} else {
> >> +		/* Only initialized buffer on stack is allowed to be accessed
> >> +		 * with variable offset. With uninitialized buffer it's hard to
> >> +		 * guarantee that whole memory is marked as initialized on
> >> +		 * helper return since specific bounds are unknown what may
> >> +		 * cause uninitialized stack leaking.
> >> +		 */
> >> +		if (meta && meta->raw_mode) {
> >> +			char tn_buf[48];
> >> +
> >> +			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
> >> +			verbose(env, "R%d invalid indirect access to uninitialized stack var_off=%s\n",
> >> +				regno, tn_buf);
> >> +			return -EACCES;
> >> +		}
> > 
> > Hmm, I think we should probably handle this in similar way like we do
> > in case of variable stack access when it comes to stack size:
> > 
> >                if (!tnum_is_const(reg->var_off))
> >                         /* For unprivileged variable accesses, disable raw
> >                          * mode so that the program is required to
> >                          * initialize all the memory that the helper could
> >                          * just partially fill up.
> >                          */
> >                         meta = NULL;
> > 
> > So we error out naturally on the loop later where we also mark for
> > liveness, and also allow for more flexibility if we know stack must
> > already be initialized in this range.
> > 
> >>  		min_off = reg->smin_value + reg->off;
> >>  		max_off = reg->umax_value + reg->off;
> >>  		err = __check_stack_boundary(env, regno, min_off, access_size,
> 
> Btw, shouldn't above two additions be sanity checked for wrap-around
> resp. truncation?

Good question.

As I can see, both reg->smin_value and reg->off are checked by
check_reg_sane_offset() in adjust_ptr_min_max_vals() that handles
pointer arithmetics. And I don't know how to come up with variable
offset w/o pointer arithmetics, i.e. these both should be in
(-BPF_MAX_VAR_OFF; BPF_MAX_VAR_OFF).

As for reg->umax_value, I see that it's checked in check_func_arg()
before calling to check_helper_mem_access() (that in turn calls to
check_stack_boundary()):

		if (reg->umax_value >= BPF_MAX_VAR_SIZ) {
			verbose(env, "R%d unbounded memory access, use 'var &= const' or 'if (var < const)'\n",
                        	regno);
			return -EACCES;
		}

So my understanding is with all these checks that happen beforehand,
there should not be overflow and int is used for offset in both the old
code, that handles constant offset, and this new code for variable
offset.

Let me know if I'm missing something.

> >> @@ -2225,12 +2244,6 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
> >>  			return err;
> >>  	}
> >>  
> >> -	if (meta && meta->raw_mode) {
> >> -		meta->access_size = access_size;
> >> -		meta->regno = regno;
> >> -		return 0;
> >> -	}
> > 
> > This can then also stay as-is.
> > 
> >>  	for (i = min_off; i < max_off + access_size; i++) {
> >>  		u8 *stype;
> >>  
> >>
> > 
> 

-- 
Andrey Ignatov