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

[Kirill Tkhai]

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:

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;