Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return probe

[Jiri Olsa]

On Wed, Apr 03, 2024 at 07:00:07PM -0700, Andrii Nakryiko wrote:

SNIP

> Check rt_sigreturn syscall (manpage at [0], for example).
> 
>        sigreturn() exists only to allow the implementation of signal
>        handlers.  It should never be called directly.  (Indeed, a simple
>        sigreturn() wrapper in the GNU C library simply returns -1, with
>        errno set to ENOSYS.)  Details of the arguments (if any) passed
>        to sigreturn() vary depending on the architecture.  (On some
>        architectures, such as x86-64, sigreturn() takes no arguments,
>        since all of the information that it requires is available in the
>        stack frame that was previously created by the kernel on the
>        user-space stack.)
> 
> This is a very similar use case. Also, check its source code in
> arch/x86/kernel/signal_64.c. It sends SIGSEGV to the calling process
> on any sign of something not being right. It's exactly the same with
> sys_uretprobe.
> 
>   [0] https://man7.org/linux/man-pages/man2/sigreturn.2.html
> 
> > And the number of syscalls are limited resource.
> 
> We have almost 500 of them, it didn't seems like adding 1-2 for good
> reasons would be a problem. Can you please point to where the limits
> on syscalls as a resource are described? I'm curious to learn.
> 
> >
> > I'm actually not sure how much we need to care of it, but adding a new
> > syscall is worth to be discussed carefully because all of them are
> > user-space compatibility.
> 
> Absolutely, it's a good discussion to have.
> 
> >
> > > > > > Also, we should run syzkaller on this syscall. And if uretprobe is
> > > > >
> > > > > right, I'll check on syzkaller
> > > > >
> > > > > > set in the user function, what happen if the user function directly
> > > > > > calls this syscall? (maybe it consumes shadow stack?)
> > > > >
> > > > > the process should receive SIGILL if there's no pending uretprobe for
> > > > > the current task, or it will trigger uretprobe if there's one pending
> > > >
> > > > No, that is too aggressive and not safe. Since the syscall is exposed to
> > > > user program, it should return appropriate error code instead of SIGILL.
> > > >
> > >
> > > This is the way it is today with uretprobes even through interrupt.
> >
> > I doubt that the interrupt (exception) and syscall should be handled
> > differently. Especially, this exception is injected by uprobes but
> > syscall will be caused by itself. But syscall can be called from user
> > program (of couse this works as sys_kill(self, SIGILL)).
> 
> Yep, I'd keep the behavior the same between uretprobes implemented
> through int3 and sys_uretprobe.

+1 

> 
> >
> > > E.g., it could happen that user process is using fibers and is
> > > replacing stack pointer without kernel realizing this, which will
> > > trigger some defensive checks in uretprobe handling code and kernel
> > > will send SIGILL because it can't support such cases. This is
> > > happening today already, and it works fine in practice (except for
> > > applications that manually change stack pointer, too bad, you can't
> > > trace them with uretprobes, unfortunately).
> >
> > OK, we at least need to document it.
> 
> +1, yep
> 
> >
> > >
> > > So I think it's absolutely adequate to have this behavior if the user
> > > process is *intentionally* abusing this API.
> >
> > Of course user expected that it is abusing. So at least we need to
> > add a document that this syscall number is reserved to uprobes and
> > user program must not use it.
> >
> 
> Totally agree about documenting this.

ok there's map page on sigreturn.. do you think we should add man page
for uretprobe or you can think of some other place to document it?

> 
> > >
> > > > >
> > > > > but we could limit the syscall to be executed just from the trampoline,
> > > > > that should prevent all the user space use cases, I'll do that in next
> > > > > version and add more tests for that
> > > >
> > > > Why not limit? :) The uprobe_handle_trampoline() expects it is called
> > > > only from the trampoline, so it is natural to check the caller address.
> > > > (and uprobe should know where is the trampoline)
> > > >
> > > > Since the syscall is always exposed to the user program, it should
> > > > - Do nothing and return an error unless it is properly called.
> > > > - check the prerequisites for operation strictly.
> > > > I concern that new system calls introduce vulnerabilities.
> > > >
> > >
> > > As Oleg and Jiri mentioned, this syscall can't harm kernel or other
> > > processes, only the process that is abusing the API. So any extra
> > > checks that would slow down this approach is an unnecessary overhead
> > > and complication that will never be useful in practice.
> >
> > I think at least it should check the caller address to ensure the
> > address is in the trampoline.
> > But anyway, uprobes itself can break the target process, so no one
> > might care if this system call breaks the process now.
> 
> If we already have an expected range of addresses, then I think it's
> fine to do a quick unlikely() check. I'd be more concerned if we need
> to do another lookup or search to just validate this. I'm sure Jiri
> will figure it out.

Oleg mentioned the trampoline address check could race with another
thread's mremap call, however trap is using that check as well, it
still seems like good idea to do it also in the uretprobe syscall

jirka