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

[Bernd Edlinger]

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.
+ */
 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;
>>>>  }
>>>>
>>>
>