Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Kirill Tkhai]

On 13.03.2020 04:05, Bernd Edlinger wrote:
> On 3/12/20 3:38 PM, Eric W. Biederman wrote:
>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
>>
>>> On 12.03.2020 15:24, Eric W. Biederman wrote:
>>>>
>>>> I actually need to switch the lock ordering here, and I haven't yet
>>>> because my son was sick yesterday.
> 
> All the best wishes to you and your son.  I hope he will get well soon.
> 
> And sorry for not missing the issue in the review.  The reason turns
> out that bprm_mm_init is called after prepare_bprm_creds, but there
> are error pathes between those where free_bprm is called up with
> cred != NULL and mm == NULL, but the mutex not locked.
> 
> I figured out a possible fix for the problem that was pointed out:
> 
> 
> From ceb6f65b52b3a7f0280f4f20509a1564a439edf6 Mon Sep 17 00:00:00 2001
> From: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
> Date: Wed, 11 Mar 2020 15:31:07 +0100
> Subject: [PATCH] Fix issues with exec_update_mutex
> 
> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
> ---
>  fs/exec.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index ffeebb1..cde4937 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1021,8 +1021,14 @@ static int exec_mmap(struct mm_struct *mm)
>  	old_mm = current->mm;
>  	exec_mm_release(tsk, old_mm);
>  
> -	if (old_mm) {
> +	if (old_mm)
>  		sync_mm_rss(old_mm);
> +
> +	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
> +	if (ret)
> +		return ret;
> +
> +	if (old_mm) {
>  		/*
>  		 * Make sure that if there is a core dump in progress
>  		 * for the old mm, we get out and die instead of going
> @@ -1032,14 +1038,11 @@ 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;
>  		}
>  	}
>  
> -	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
> -	if (ret)
> -		return ret;
> -
>  	task_lock(tsk);
>  	active_mm = tsk->active_mm;
>  	membarrier_exec_mmap(mm);
> @@ -1444,8 +1447,6 @@ static void free_bprm(struct linux_binprm *bprm)
>  {
>  	free_arg_pages(bprm);
>  	if (bprm->cred) {
> -		if (!bprm->mm)
> -			mutex_unlock(&current->signal->exec_update_mutex);
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  		abort_creds(bprm->cred);
>  	}
> @@ -1846,6 +1847,8 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	would_dump(bprm, bprm->file);
>  
>  	retval = exec_binprm(bprm);
> +	if (bprm->cred && !bprm->mm)
> +		mutex_unlock(&current->signal->exec_update_mutex);

Despite this should fix the problem, this looks like a broken puzzle.

We can't use bprm->cred as an identifier whether the mutex was locked or not.
We can check for bprm->cred in regard to cred_guard_mutex, because of there is
strong rule: "cred_guard_mutex is becomes locked together with bprm->cred assignment
(see prepare_bprm_creds()), and it becomes unlocked together with bprm->cred zeroing".
Take attention on modularity of all this: there is no dependencies between anything else.

In regard to newly introduced exec_update_mutex, your fix and source patch way look like
an obfuscation. The mutex becomes deadly glued to unrelated bprm->cred and bprm->mm,
and this introduces the problems in the future modifications and support of all involved
entities. If someone wants to move some functions in relation to each other, there will
be a pain, and this person will have to go again the same dependencies and bug way,
Eric stepped on in the original patch.