Re: [PATCH 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Tue, Mar 10, 2020 at 06:45:47PM +0100, Bernd Edlinger wrote:
>> This changes do_io_accounting to use the new exec_update_mutex
>> instead of cred_guard_mutex.
>> 
>> This fixes possible deadlocks when the trace is accessing
>> /proc/$pid/io for instance.
>> 
>> This should be safe, as the credentials are only used for reading.
>
> I'd like to see the rationale described better here for why it should be
> safe. I'm still not seeing why this is safe here, as we might check
> ptrace_may_access() with one cred and then iterate io accounting with a
> different credential...
>
> What am I missing?

The rational for non-regression is that exec_update_mutex covers all
of the same tsk->cred changes as cred_guard_mutex.  Therefore we are not
any worse off, and we avoid the deadlock.

As for safety.  Jann's argument that the only interesting credential
change is in exec applies.  All other credential changes that have any
effect on permission checks make the new cred non-dumpable (excepions
apply see the code).

So I think this is a non-regressing change.  A safe change.

I don't think either version of this code is fully correct.

Eric

>> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
>> ---
>>  fs/proc/base.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 4fdfe4f..529d0c6 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -2770,7 +2770,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
>>  	unsigned long flags;
>>  	int result;
>>  
>> -	result = mutex_lock_killable(&task->signal->cred_guard_mutex);
>> +	result = mutex_lock_killable(&task->signal->exec_update_mutex);
>>  	if (result)
>>  		return result;
>>  
>> @@ -2806,7 +2806,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
>>  	result = 0;
>>  
>>  out_unlock:
>> -	mutex_unlock(&task->signal->cred_guard_mutex);
>> +	mutex_unlock(&task->signal->exec_update_mutex);
>>  	return result;
>>  }
>>  
>> -- 
>> 1.9.1