Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Bernd Edlinger]

On 3/5/20 10:16 PM, 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 can take.
> 
> The plan is to switch the users of cred_guard_mutex to
> exed_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                    | 4 ++++
>  include/linux/sched/signal.h | 9 ++++++++-
>  kernel/fork.c                | 1 +
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index c243f9660d46..ad7b518f906d 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1182,6 +1182,7 @@ static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
>  		release_task(leader);
>  	}
>  
> +	mutex_lock(&current->signal->exec_update_mutex);
>  	bprm->unrecoverable = true;
>  	sig->group_exit_task = NULL;
>  	sig->notify_count = 0;
> @@ -1425,6 +1426,8 @@ static void free_bprm(struct linux_binprm *bprm)
>  {
>  	free_arg_pages(bprm);
>  	if (bprm->cred) {
> +		if (bprm->unrecoverable)
> +			mutex_unlock(&current->signal->exec_update_mutex);
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  		abort_creds(bprm->cred);
>  	}
> @@ -1474,6 +1477,7 @@ 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);
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 88050259c466..a29df79540ce 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/kernel/fork.c b/kernel/fork.c
> index 60a1295f4384..12896a6ecee6 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;
>  }
> 
Don't you need to add something like this to init/init_task.c ?

.exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),


Bernd.