Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Eric W. Biederman]

Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:

> On 3/5/20 10:16 PM, Eric W. Biederman wrote:
>> 
>> The cred_guard_mutex is problematic.  The cred_guard_mutex is held
>> over the userspace accesses as the arguments from userspace are read.
>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>> threads are killed.  The cred_guard_mutex is held over
>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>> 
>> Any of those can result in deadlock, as the cred_guard_mutex is held
>> over a possible indefinite userspace waits for userspace.
>> 
>> Add exec_update_mutex that is only held over exec updating process
>> with the new contents of exec, so that code that needs not to be
>> confused by exec changing the mm and the cred in ways that can not
>> happen during ordinary execution of a process can take.
>> 
>> The plan is to switch the users of cred_guard_mutex to
>> exed_udpate_mutex one by one.  This lets us move forward while still
>> being careful and not introducing any regressions.
>> 
>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@xxxxxxxxxxxxxx/
>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/
>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@xxxxxxxxxx/
>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@xxxxxxxxxx/
>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>  fs/exec.c                    | 4 ++++
>>  include/linux/sched/signal.h | 9 ++++++++-
>>  kernel/fork.c                | 1 +
>>  3 files changed, 13 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index c243f9660d46..ad7b518f906d 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1182,6 +1182,7 @@ static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
>>  		release_task(leader);
>>  	}
>>  
>> +	mutex_lock(&current->signal->exec_update_mutex);
>>  	bprm->unrecoverable = true;
>>  	sig->group_exit_task = NULL;
>>  	sig->notify_count = 0;
>> @@ -1425,6 +1426,8 @@ static void free_bprm(struct linux_binprm *bprm)
>>  {
>>  	free_arg_pages(bprm);
>>  	if (bprm->cred) {
>> +		if (bprm->unrecoverable)
>> +			mutex_unlock(&current->signal->exec_update_mutex);
>>  		mutex_unlock(&current->signal->cred_guard_mutex);
>>  		abort_creds(bprm->cred);
>>  	}
>> @@ -1474,6 +1477,7 @@ void install_exec_creds(struct linux_binprm *bprm)
>>  	 * credentials; any time after this it may be unlocked.
>>  	 */
>>  	security_bprm_committed_creds(bprm);
>> +	mutex_unlock(&current->signal->exec_update_mutex);
>>  	mutex_unlock(&current->signal->cred_guard_mutex);
>>  }
>>  EXPORT_SYMBOL(install_exec_creds);
>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>> index 88050259c466..a29df79540ce 100644
>> --- a/include/linux/sched/signal.h
>> +++ b/include/linux/sched/signal.h
>> @@ -224,7 +224,14 @@ struct signal_struct {
>>  
>>  	struct mutex cred_guard_mutex;	/* guard against foreign influences on
>>  					 * credential calculations
>> -					 * (notably. ptrace) */
>> +					 * (notably. ptrace)
>> +					 * Deprecated do not use in new code.
>> +					 * Use exec_update_mutex instead.
>> +					 */
>> +	struct mutex exec_update_mutex;	/* Held while task_struct is being
>> +					 * updated during exec, and may have
>> +					 * inconsistent permissions.
>> +					 */
>>  } __randomize_layout;
>>  
>>  /*
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 60a1295f4384..12896a6ecee6 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1594,6 +1594,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>>  	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
>>  
>>  	mutex_init(&sig->cred_guard_mutex);
>> +	mutex_init(&sig->exec_update_mutex);
>>  
>>  	return 0;
>>  }
>> 
> Don't you need to add something like this to init/init_task.c ?
>
> .exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),

Yes.  I overlooked that.  Thank you.

Eric