Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

[Tycho Andersen]

On Thu, Nov 30, 2023 at 02:00:01PM -0500, Mathieu Desnoyers wrote:
> On 2023-11-30 13:54, Tycho Andersen wrote:
> > On Thu, Nov 30, 2023 at 07:37:02PM +0100, Florian Weimer wrote:
> > > * Tycho Andersen:
> > > 
> > > > From: Tycho Andersen <tandersen@xxxxxxxxxxx>
> > > > 
> > > > We are using the pidfd family of syscalls with the seccomp userspace
> > > > notifier. When some thread triggers a seccomp notification, we want to do
> > > > some things to its context (munge fd tables via pidfd_getfd(), maybe write
> > > > to its memory, etc.). However, threads created with ~CLONE_FILES or
> > > > ~CLONE_VM mean that we can't use the pidfd family of syscalls for this
> > > > purpose, since their fd table or mm are distinct from the thread group
> > > > leader's. In this patch, we relax this restriction for pidfd_open().
> > > 
> > > Does this mean that pidfd_getfd cannot currently be used to get
> > > descriptors for a TID if that TID doesn't happen to share its descriptor
> > > set with the thread group leader?
> > 
> > Correct, that's what I'm trying to solve.
> > 
> > > I'd like to offer a userspace API which allows safe stashing of
> > > unreachable file descriptors on a service thread.
> > 
> > By "safe" here do you mean not accessible via pidfd_getfd()?
> 
> For the LTTng-UST use-case, we need to be able to create and
> use a file descriptor from an agent thread injected within the target
> process in a way that is safe against patterns where the application
> blindly close all file descriptors (for-loop doing close(2),
> closefrom(2) or closeall(2)).
> 
> The main issue here is that even though we could handle errors
> (-1, errno=EBADF) in the sendmsg/recvmsg calls, re-use of a file
> descriptor by the application can lead to data corruption, which
> is certainly an unwanted consequence.
> 
> AFAIU glibc has similar requirements with respect to io_uring
> file descriptors.

I see, thanks. And this introduces another problem: what if one of
these things is a memfd, then that memory needs to be invisible to the
process as well presumably?

This "invisible to the process" mapping would solve another
longstanding problem with seccomp: handlers could copy syscall
arguments to this safe memory area and then _CONTINUE things safely...

Tycho