Re: [PATCH bpf] bpf: Fix tr dereferencing

[Leon Hwang <hffilwlqm@gmail.com>]

On 15/9/23 10:13, Hengqi Chen wrote:
> On Thu, Sep 14, 2023 at 10:51 PM Leon Hwang <hffilwlqm@gmail.com> wrote:
>>
>> Fix 'tr' dereferencing bug when CONFIG_BPF_JIT is turned off.
>>
>> Like 'bpf_trampoline_get_progs()', return 'ERR_PTR()' and then check by
>> 'IS_ERR()'. As a result, when CONFIG_BPF_JIT is turned off, it's able to
>> handle the case that 'bpf_trampoline_get()' returns
>> 'ERR_PTR(-EOPNOTSUPP)'.
>>
>> Fixes: 4a1e7c0c63e0 ("bpf: Support attaching freplace programs to multiple attach points")
>> Fixes: f7b12b6fea00 ("bpf: verifier: refactor check_attach_btf_id()")
>> Fixes: 69fd337a975c ("bpf: per-cgroup lsm flavor")
>> Reported-by: kernel test robot <lkp@intel.com>
>> Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
>> Closes: https://lore.kernel.org/r/202309131936.5Nc8eUD0-lkp@intel.com/
>> Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
>> ---
>>  kernel/bpf/syscall.c    | 4 ++--
>>  kernel/bpf/trampoline.c | 6 +++---
>>  kernel/bpf/verifier.c   | 4 ++--
>>  3 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 6a692f3bea150..5748d01c99854 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -3211,8 +3211,8 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>>                 }
>>
>>                 tr = bpf_trampoline_get(key, &tgt_info);
>> -               if (!tr) {
>> -                       err = -ENOMEM;
>> +               if (IS_ERR(tr)) {
>> +                       err = PTR_ERR(tr);
>>                         goto out_unlock;
> 
> IS_ERR does not check the null case, so this should be IS_ERR_OR_NULL instead.

Actually, bpf_trampoline_get() would not return NULL. It returns ERR_PTR(-ENOMEM) 
or a valid ptr.

Thanks,
Leon

> 
>>                 }
>>         } else {
>> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
>> index e97aeda3a86b5..1952614778433 100644
>> --- a/kernel/bpf/trampoline.c
>> +++ b/kernel/bpf/trampoline.c
>> @@ -697,8 +697,8 @@ int bpf_trampoline_link_cgroup_shim(struct bpf_prog *prog,
>>
>>         bpf_lsm_find_cgroup_shim(prog, &bpf_func);
>>         tr = bpf_trampoline_get(key, &tgt_info);
>> -       if (!tr)
>> -               return  -ENOMEM;
>> +       if (IS_ERR(tr))
>> +               return PTR_ERR(tr);
>>
>>         mutex_lock(&tr->mutex);
>>
>> @@ -775,7 +775,7 @@ struct bpf_trampoline *bpf_trampoline_get(u64 key,
>>
>>         tr = bpf_trampoline_lookup(key);
>>         if (!tr)
>> -               return NULL;
>> +               return ERR_PTR(-ENOMEM);
>>
>>         mutex_lock(&tr->mutex);
>>         if (tr->func.addr)
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 18e673c0ac159..054063ead0e54 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -19771,8 +19771,8 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>>
>>         key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
>>         tr = bpf_trampoline_get(key, &tgt_info);
>> -       if (!tr)
>> -               return -ENOMEM;
>> +       if (IS_ERR(tr))
>> +               return PTR_ERR(tr);
>>
>>         if (tgt_prog && tgt_prog->aux->tail_call_reachable)
>>                 tr->flags = BPF_TRAMP_F_TAIL_CALL_CTX;
>>
>> base-commit: cbb1dbcd99b0ae74c45c4c83c6d213c12c31785c
>> --
>> 2.41.0
>>