Re: [PATCH] bpf: do not restore dst_reg when cur_state is freed

[Yu Xu]

On 3/21/19 4:50 PM, Daniel Borkmann wrote:
> On 03/21/2019 09:31 AM, Xu Yu wrote:
> > Syzkaller hit 'KASAN: use-after-free Write in sanitize_ptr_alu' bug.
> > Call trace:
> >    dump_stack+0xbf/0x12e
> >    print_address_description+0x6a/0x280
> >    kasan_report+0x237/0x360
> >    sanitize_ptr_alu+0x85a/0x8d0
> >    adjust_ptr_min_max_vals+0x8f2/0x1ca0
> >    adjust_reg_min_max_vals+0x8ed/0x22e0
> >    do_check+0x1ca6/0x5d00
> >    bpf_check+0x9ca/0x2570
> >    bpf_prog_load+0xc91/0x1030
> >    __se_sys_bpf+0x61e/0x1f00
> >    do_syscall_64+0xc8/0x550
> >    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > Fault injection trace:
> >   Âkfree+0xea/0x290
> >   Âfree_func_state+0x4a/0x60
> >   Âfree_verifier_state+0x61/0xe0
> >   Âpush_stack+0x216/0x2f0	          <- inject failslab
> >   Âsanitize_ptr_alu+0x2b1/0x8d0
> >   Âadjust_ptr_min_max_vals+0x8f2/0x1ca0
> >   Âadjust_reg_min_max_vals+0x8ed/0x22e0
> >   Âdo_check+0x1ca6/0x5d00
> >   Âbpf_check+0x9ca/0x2570
> >   Âbpf_prog_load+0xc91/0x1030
> >   Â__se_sys_bpf+0x61e/0x1f00
> >   Âdo_syscall_64+0xc8/0x550
> >   Âentry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > When kzalloc() fails in push_stack(), free_verifier_state() will free
> > current verifier state. As push_stack() returns, dst_reg was restored
> > if ptr_is_dst_reg is false. However, as member of the cur_state, dst_reg
> > is also freed, and error occurs when dereferencing dst_reg.
> >
> > Simply fix it by checking whether cur_state is NULL before retoring
> > dst_reg.
> >
> > Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
> > ---
> >   kernel/bpf/verifier.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index ce166a0..018ce4f 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3368,7 +3368,7 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
> >   		*dst_reg = *ptr_reg;
> >   	}
> >   	ret = push_stack(env, env->insn_idx + 1, env->insn_idx, true);
> > -	if (!ptr_is_dst_reg)
> > +	if (!ptr_is_dst_reg && env->cur_state)
> >   		*dst_reg = tmp;
>
> Good catch, test should be more obvious rewritten as:
>
>    if (!ptr_is_dst_reg && ret)
>
> Could you resubmit with that?

sure, will send patch v2 later.

thanks,
Yu

> >   	return !ret ? -EFAULT : 0;
> >   }
>
> Thanks,
> Daniel