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

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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?

>  	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;
>  }
> 




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