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Subject: Re: [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix
 deadlocks in execve
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
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        Ingo Molnar <mingo@kernel.org>,
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From:   Bernd Edlinger <bernd.edlinger@hotmail.de>
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On 3/10/20 8:01 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
> 
>> This changes kcmp_epoll_target to use the new exec_update_mutex
>> instead of cred_guard_mutex.
>>
>> This should be safe, as the credentials are only used for reading,
>> and furthermore ->mm and ->sighand are updated on execve,
>> but only under the new exec_update_mutex.
>>
> 
> Can you add a comment that the exec_update_mutex is not needed for
> KCMP_FILE?  As both sets of credentials during exec are valid
> for accessing the files so exec_update_mutex does not matter.
> 

some files are closed by do_close_on_exec,
so in theory this allows you to examine files that
were open in the old user but closed for the new user
with either credential.

It is not a race condition, but it may be a security
concern.

> I don't think exec_update_mutex is needed for KCMP_SYSVSEM
> or KCMP_EPOLL_TFD either.  As I don't think exec changes either
> one of those.
> 

KCMP_EPOLL_TFD is also accessing file pointers,
that is possible.

It might be that KCMP_SYSVSEM is a missed optimization, but
I may have overlooked something.
I'd rather err on the safe side.

> Eric
> 
> 
>> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
>> ---
>>  kernel/kcmp.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/kcmp.c b/kernel/kcmp.c
>> index a0e3d7a..b3ff928 100644
>> --- a/kernel/kcmp.c
>> +++ b/kernel/kcmp.c
>> @@ -173,8 +173,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>>  	/*
>>  	 * One should have enough rights to inspect task details.
>>  	 */
>> -	ret = kcmp_lock(&task1->signal->cred_guard_mutex,
>> -			&task2->signal->cred_guard_mutex);
>> +	ret = kcmp_lock(&task1->signal->exec_update_mutex,
>> +			&task2->signal->exec_update_mutex);
>>  	if (ret)
>>  		goto err;
>>  	if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
>> @@ -229,8 +229,8 @@ static int kcmp_epoll_target(struct task_struct *task1,
>>  	}
>>  
>>  err_unlock:
>> -	kcmp_unlock(&task1->signal->cred_guard_mutex,
>> -		    &task2->signal->cred_guard_mutex);
>> +	kcmp_unlock(&task1->signal->exec_update_mutex,
>> +		    &task2->signal->exec_update_mutex);
>>  err:
>>  	put_task_struct(task1);
>>  	put_task_struct(task2);