Re: [PATCH v3 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Bernd Edlinger]

On 3/17/20 9:56 AM, Kirill Tkhai wrote:
> On 14.03.2020 12:11, Bernd Edlinger 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.
>>
>> The plan is to switch the users of cred_guard_mutex to
>> exec_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>
>> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
>> ---
>>  fs/exec.c                    | 17 ++++++++++++++---
>>  include/linux/binfmts.h      |  8 +++++++-
>>  include/linux/sched/signal.h |  9 ++++++++-
>>  init/init_task.c             |  1 +
>>  kernel/fork.c                |  1 +
>>  5 files changed, 31 insertions(+), 5 deletions(-)
>>
>> v3: this update fixes lock-order and adds an explicit data member in linux_binprm
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index d820a72..11974a1 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1014,12 +1014,17 @@ static int exec_mmap(struct mm_struct *mm)
>>  {
>>  	struct task_struct *tsk;
>>  	struct mm_struct *old_mm, *active_mm;
>> +	int ret;
>>  
>>  	/* Notify parent that we're no longer interested in the old VM */
>>  	tsk = current;
>>  	old_mm = current->mm;
>>  	exec_mm_release(tsk, old_mm);
>>  
>> +	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>> +	if (ret)
>> +		return ret;
>> +
>>  	if (old_mm) {
>>  		sync_mm_rss(old_mm);
>>  		/*
>> @@ -1031,9 +1036,11 @@ static int exec_mmap(struct mm_struct *mm)
>>  		down_read(&old_mm->mmap_sem);
>>  		if (unlikely(old_mm->core_state)) {
>>  			up_read(&old_mm->mmap_sem);
>> +			mutex_unlock(&tsk->signal->exec_update_mutex);
>>  			return -EINTR;
>>  		}
>>  	}
>> +
>>  	task_lock(tsk);
>>  	active_mm = tsk->active_mm;
>>  	membarrier_exec_mmap(mm);
>> @@ -1288,11 +1295,12 @@ int flush_old_exec(struct linux_binprm * bprm)
>>  		goto out;
>>  
>>  	/*
>> -	 * After clearing bprm->mm (to mark that current is using the
>> -	 * prepared mm now), we have nothing left of the original
>> +	 * After setting bprm->called_exec_mmap (to mark that current is
>> +	 * using the prepared mm now), we have nothing left of the original
>>  	 * process. If anything from here on returns an error, the check
>>  	 * in search_binary_handler() will SEGV current.
>>  	 */
>> +	bprm->called_exec_mmap = 1;
> 
> The two below is non-breaking pair:
> 
> exec_mmap(bprm->mm);
> bprm->called_exec_mmap = 1;
> 
> Why not move this into exec_mmap(), so nobody definitely inserts something
> between them?
> 

Hmm, could be done, but then I would probably need a different name than
"called_exec_mmap".

How about adding a nice function comment to exec_mmap that calls out the
changed behaviour that the exec_update_mutex is taken unless the function
fails?


Bernd.


>>  	bprm->mm = NULL;
>>  
>>  #ifdef CONFIG_POSIX_TIMERS
>> @@ -1438,6 +1446,8 @@ static void free_bprm(struct linux_binprm *bprm)
>>  {
>>  	free_arg_pages(bprm);
>>  	if (bprm->cred) {
>> +		if (bprm->called_exec_mmap)
>> +			mutex_unlock(&current->signal->exec_update_mutex);
>>  		mutex_unlock(&current->signal->cred_guard_mutex);
>>  		abort_creds(bprm->cred);
>>  	}
>> @@ -1487,6 +1497,7 @@ 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);
>> @@ -1678,7 +1689,7 @@ int search_binary_handler(struct linux_binprm *bprm)
>>  
>>  		read_lock(&binfmt_lock);
>>  		put_binfmt(fmt);
>> -		if (retval < 0 && !bprm->mm) {
>> +		if (retval < 0 && bprm->called_exec_mmap) {
>>  			/* we got to flush_old_exec() and failed after it */
>>  			read_unlock(&binfmt_lock);
>>  			force_sigsegv(SIGSEGV);
>> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
>> index b40fc63..a345d9f 100644
>> --- a/include/linux/binfmts.h
>> +++ b/include/linux/binfmts.h
>> @@ -44,7 +44,13 @@ struct linux_binprm {
>>  		 * exec has happened. Used to sanitize execution environment
>>  		 * and to set AT_SECURE auxv for glibc.
>>  		 */
>> -		secureexec:1;
>> +		secureexec:1,
>> +		/*
>> +		 * Set by flush_old_exec, when exec_mmap has been called.
>> +		 * This is past the point of no return, when the
>> +		 * exec_update_mutex has been taken.
>> +		 */
>> +		called_exec_mmap:1;
>>  #ifdef __alpha__
>>  	unsigned int taso:1;
>>  #endif
>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>> index 8805025..a29df79 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/init/init_task.c b/init/init_task.c
>> index 9e5cbe5..bd403ed 100644
>> --- a/init/init_task.c
>> +++ b/init/init_task.c
>> @@ -26,6 +26,7 @@
>>  	.multiprocess	= HLIST_HEAD_INIT,
>>  	.rlim		= INIT_RLIMITS,
>>  	.cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex),
>> +	.exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),
>>  #ifdef CONFIG_POSIX_TIMERS
>>  	.posix_timers = LIST_HEAD_INIT(init_signals.posix_timers),
>>  	.cputimer	= {
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 8642530..036b692 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;
>>  }
>>
>