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

[Eric W. Biederman]

Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:

> On 09.03.2020 00:38, 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.
>> 
>> 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>
>> ---
>>  fs/exec.c                    | 9 +++++++++
>>  include/linux/sched/signal.h | 9 ++++++++-
>>  init/init_task.c             | 1 +
>>  kernel/fork.c                | 1 +
>>  4 files changed, 19 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index d820a7272a76..ffeebb1f167b 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1014,6 +1014,7 @@ 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;
>> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
>>  			return -EINTR;
>>  		}
>>  	}
>> +
>> +	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>> +	if (ret)
>> +		return ret;
>
> You missed old_mm->mmap_sem unlock. See here:

Duh.  Thank you.

I actually need to switch the lock ordering here, and I haven't yet
because my son was sick yesterday.

Something like this.

diff --git a/fs/exec.c b/fs/exec.c
index 96f89401b4d1..03d50c27ec01 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1020,9 +1020,14 @@ static int exec_mmap(struct mm_struct *mm)
        tsk = current;
        old_mm = current->mm;
        exec_mm_release(tsk, 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) {
-               sync_mm_rss(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 +1037,11 @@ 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);



> diff --git a/fs/exec.c b/fs/exec.c
> index 47582cd97f86..d557bac3e862 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1063,8 +1063,11 @@ static int exec_mmap(struct mm_struct *mm)
>  	}
>  
>  	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
> -	if (ret)
> +	if (ret) {
> +		if (old_mm)
> +			up_read(&old_mm->mmap_sem);
>  		return ret;
> +	}
>  
>  	task_lock(tsk);
>  	active_mm = tsk->active_mm;

Eric