linux-kernel - Re: [PATCH] seccomp: passthrough uretprobe systemcall without filtering

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Message-ID: <20250121152843.GC3422@redhat.com>
Date: Tue, 21 Jan 2025 16:28:43 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Kees Cook <kees@...nel.org>
Cc: Eyal Birger <eyal.birger@...il.com>, luto@...capital.net,
	wad@...omium.org, ldv@...ace.io, mhiramat@...nel.org,
	andrii@...nel.org, jolsa@...nel.org, alexei.starovoitov@...il.com,
	olsajiri@...il.com, cyphar@...har.com, songliubraving@...com,
	yhs@...com, john.fastabend@...il.com, peterz@...radead.org,
	tglx@...utronix.de, bp@...en8.de, daniel@...earbox.net,
	ast@...nel.org, andrii.nakryiko@...il.com, rostedt@...dmis.org,
	rafi@....io, shmulik.ladkani@...il.com, bpf@...r.kernel.org,
	linux-api@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] seccomp: passthrough uretprobe systemcall without
 filtering

On 01/20, Kees Cook wrote:
>
> > The only difference is that sys_uretprobe() is new and existing setups
> > doesn't know about it. Suppose you have
> >
> > 	int func(void)
> > 	{
> > 		return 123;
> > 	}
> >
> > 	int main(void)
> > 	{
> > 		seccomp(SECCOMP_SET_MODE_STRICT, 0,0);
> > 		for (;;)
> > 			func();
> > 	}
> >
> > and it runs with func() uretprobed.
> >
> > If you install the new kernel, this application will crash immediately.
> >
> > I understand your objections, but what do you think we can do instead?
> > I don't think a new "try_to_speedup_uretprobes_at_your_own_risk" sysctl
> > makes sense, it will be almost never enabled...
>
> This seems like a uretprobes design problem. If it's going to use
> syscalls, it must take things like seccomp into account.

True. I reviewed that patch, and I forgot about seccomp too.

> SECCOMP_SET_MODE_STRICT will also crash in the face of syscall_restart...

Yes, I guess SECCOMP_SET_MODE_STRICT assumes that read/write can't return
ERESTART_RESTARTBLOCK.

But again, what can we do right now?

I do not like the idea to revert the patch which adds sys_uretprobe().
Don't get me wrong, I do not use uprobes, so personally I don't really
care about the performance improvements it adds. Not to mention FRED,
although I have no idea when it will be available.

Lets forget about sys_uretprobe(). Lets suppose the kernel doesn't have
ERESTART_RESTARTBLOCK/sys_restart_syscall and we want to add this feature
today.

How do you think we can do this without breaking the existing setups which
use seccomp ?

Oleg.