Date:   Tue, 3 Mar 2020 18:01:09 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.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: [PATCHv5] exec: Fix a deadlock in ptrace

On Tue, Mar 03, 2020 at 04:48:01PM +0000, Bernd Edlinger wrote:
> On 3/3/20 4:18 PM, Eric W. Biederman wrote:
> > Bernd Edlinger <bernd.edlinger@...mail.de> writes:
> > 
> >> This fixes a deadlock in the tracer when tracing a multi-threaded
> >> application that calls execve while more than one thread are running.
> >>
> >> I observed that when running strace on the gcc test suite, it always
> >> blocks after a while, when expect calls execve, because other threads
> >> have to be terminated.  They send ptrace events, but the strace is no
> >> longer able to respond, since it is blocked in vm_access.
> >>
> >> The deadlock is always happening when strace needs to access the
> >> tracees process mmap, while another thread in the tracee starts to
> >> execve a child process, but that cannot continue until the
> >> PTRACE_EVENT_EXIT is handled and the WIFEXITED event is received:
> > 
> > A couple of things.
> > 
> > Why do we think it is safe to change the behavior exposed to userspace?
> > Not the deadlock but all of the times the current code would not
> > deadlock?
> > 
> > Especially given that this is a small window it might be hard for people
> > to track down and report so we need a strong argument that this won't
> > break existing userspace before we just change things.
> > 
> 
> Hmm, I tend to agree.
> 
> > Usually surveying all of the users of a system call that we can find
> > and checking to see if they might be affected by the change in behavior
> > is difficult enough that we usually opt for not being lazy and
> > preserving the behavior.
> > 
> > This patch is up to two changes in behavior now, that could potentially
> > affect a whole array of programs.  Adding linux-api so that this change
> > in behavior can be documented if/when this change goes through.
> > 
> 
> One is PTRACE_ACCESS possibly returning EAGAIN, yes.
> 
> We could try to restrict that behavior change to when any
> thread is ptraced when execve starts, can't be too complicated.
> 
> 
> But the other is only SYS_seccomp returning EAGAIN, when a different
> thread of the current process is calling execve at the same time.
> 
> I would consider it completely impossible to have any user-visual effect,
> since de_thread is just terminating all threads, including the thread
> where the -EAGAIN was returned, so we will never know what happened.

I think if we risk a user-space facing change we should try the simple
thing first before making the fix more convoluted? But it's a tough
call...

> 
> 
> > If you can split the documentation and test fixes out into separate
> > patches that would help reviewing this code, or please make it explicit
> > that the your are changing documentation about behavior that is changing
> > with this patch.
> > 
> 
> I am not sure if I have touched the right user documentation.
> 
> I only saw a document referring to a non-existent "current->cred_replace_mutex"
> I haven't digged the git history, but that must be pre-historic IMHO.
> It appears to me that is some developer documentation, but it's nevertheless
> worth to keep up to date when the code changes.
> 
> So where would I add the possibility for PTRACE_ATTACH to return -EAGAIN ?

Since that would be a potentially user-visible change it would make the
most sense to add it to man ptrace(2) if/when we land this change.

For developers, placing a comment in kernel/ptrace.c:ptrace_attach()
would make the most sense? We already have something about exec
protection in there.

Christian

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