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

[Kirill Tkhai]

On 18.03.2020 23:06, Bernd Edlinger wrote:
> On 3/18/20 1:22 PM, Kirill Tkhai wrote:
>> On 18.03.2020 00:53, Bernd Edlinger wrote:
>>> 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?
>>
>> Not sure, I understand correct.
>>
>> Could you post this like a small patch hunk (on top of anything you want)?
>>
> 
> I was thinking of something like that:
> 
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1010,6 +1010,11 @@ ssize_t read_code(struct file *file, unsigned long addr, 
>  }
>  EXPORT_SYMBOL(read_code);
>  
> +/*
> + * Maps the mm_struct mm into the current task struct.
> + * On success, this function returns with the mutex
> + * exec_update_mutex locked.
> + */

Looks OK for me.

>  static int exec_mmap(struct mm_struct *mm)
>  {
>         struct task_struct *tsk;
> 
> 
>>> 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;
>>>>>  }
>>>>>
>>>>
>>