linux-kernel - Re: [PATCH v4 2/3] binder: use cred instead of task for getsecid

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Date:   Mon, 11 Oct 2021 14:59:13 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Paul Moore <paul@...l-moore.com>, Todd Kjos <tkjos@...gle.com>
Cc:     gregkh@...uxfoundation.org, arve@...roid.com, tkjos@...roid.com,
        maco@...roid.com, christian@...uner.io,
        James Morris <jmorris@...ei.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>, keescook@...omium.org,
        jannh@...gle.com, Jeffrey Vander Stoep <jeffv@...gle.com>,
        zohar@...ux.ibm.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, joel@...lfernandes.org,
        kernel-team@...roid.com, kernel test robot <lkp@...el.com>,
        stable@...r.kernel.org, Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v4 2/3] binder: use cred instead of task for getsecid

On 10/11/2021 2:33 PM, Paul Moore wrote:
> On Wed, Oct 6, 2021 at 8:46 PM Todd Kjos <tkjos@...gle.com> wrote:
>> Use the 'struct cred' saved at binder_open() to lookup
>> the security ID via security_cred_getsecid(). This
>> ensures that the security context that opened binder
>> is the one used to generate the secctx.
>>
>> Fixes: ec74136ded79 ("binder: create node flag to request sender's
>> security context")
>> Signed-off-by: Todd Kjos <tkjos@...gle.com>
>> Suggested-by: Stephen Smalley <stephen.smalley.work@...il.com>
>> Reported-by: kernel test robot <lkp@...el.com>
>> Cc: stable@...r.kernel.org # 5.4+
>> ---
>> v3: added this patch to series
>> v4: fix build-break for !CONFIG_SECURITY
>>
>>  drivers/android/binder.c | 11 +----------
>>  include/linux/security.h |  4 ++++
>>  2 files changed, 5 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
>> index ca599ebdea4a..989afd0804ca 100644
>> --- a/drivers/android/binder.c
>> +++ b/drivers/android/binder.c
>> @@ -2722,16 +2722,7 @@ static void binder_transaction(struct binder_proc *proc,
>>                 u32 secid;
>>                 size_t added_size;
>>
>> -               /*
>> -                * Arguably this should be the task's subjective LSM secid but
>> -                * we can't reliably access the subjective creds of a task
>> -                * other than our own so we must use the objective creds, which
>> -                * are safe to access.  The downside is that if a task is
>> -                * temporarily overriding it's creds it will not be reflected
>> -                * here; however, it isn't clear that binder would handle that
>> -                * case well anyway.
>> -                */
>> -               security_task_getsecid_obj(proc->tsk, &secid);
>> +               security_cred_getsecid(proc->cred, &secid);
>>                 ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
>>                 if (ret) {
>>                         return_error = BR_FAILED_REPLY;
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index 6344d3362df7..f02cc0211b10 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -1041,6 +1041,10 @@ static inline void security_transfer_creds(struct cred *new,
>>  {
>>  }
>>
>> +static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
>> +{
>> +}
> Since security_cred_getsecid() doesn't return an error code we should
> probably set the secid to 0 in this case, for example:
>
>   static inline void security_cred_getsecid(...)
>   {
>     *secid = 0;
>   }

If CONFIG_SECURITY is unset there shouldn't be any case where
the secid value is ever used for anything. Are you suggesting that
it be set out of an abundance of caution?