linux-kernel - Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard

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Message-ID: <f37a5d68-9674-533f-ee9c-a49174605710@virtuozzo.com>
Date:   Thu, 12 Mar 2020 13:27:30 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex

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@dhcp22.suse.cz/
> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
> 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@...ssion.com>
> ---
>  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;