Re: [PATCHv5] exec: Fix a deadlock in ptrace

[Christian Brauner]

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@xxxxxxxxxx> 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