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

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Date: Wed, 3 Apr 2024 16:49:10 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Jiri Olsa <olsajiri@...il.com>, Steven Rostedt <rostedt@...dmis.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
	Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org
Subject: Re: [PATCHv2 1/3] uprobe: Add uretprobe syscall to speed up return
 probe

Again, I leave this to you and Jiri, but

On 04/03, Masami Hiramatsu wrote:
>
> On Wed, 3 Apr 2024 11:47:41 +0200
> > > 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.

..

> 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.

We have sys_munmap(). should it check if the caller is going to unmap
the code region which contains regs->ip and do nothing?

I don't think it should. Userspace should blame itself, SIGSEGV is not
"too aggressive" in this case.

> I concern that new system calls introduce vulnerabilities.

Yes, we need to ensure that sys_uretprobe() can only damage the malicious
caller and nothing else.

Oleg.