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SubjectRe: [PATCH] bpf: do not restore dst_reg when cur_state is freed
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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:
>>  kfree+0xea/0x290
>>  free_func_state+0x4a/0x60
>>  free_verifier_state+0x61/0xe0
>>  push_stack+0x216/0x2f0 <- inject failslab
>>  sanitize_ptr_alu+0x2b1/0x8d0
>>  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
>>
>> 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@linux.alibaba.com>
>> ---
>> 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
>

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