linux-kernel - Re: uprobes are destructive but exposed by perf under CAP

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Message-ID: <20250702115814.GA1099709@noisy.programming.kicks-ass.net>
Date: Wed, 2 Jul 2025 13:58:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jann Horn <jannh@...gle.com>
Cc: Serge Hallyn <serge@...lyn.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org,
	Kernel Hardening <kernel-hardening@...ts.openwall.com>,
	linux-hardening@...r.kernel.org,
	kernel list <linux-kernel@...r.kernel.org>,
	Alexey Budankov <alexey.budankov@...ux.intel.com>,
	James Morris <jamorris@...ux.microsoft.com>
Subject: Re: uprobes are destructive but exposed by perf under CAP_PERFMON

On Tue, Jul 01, 2025 at 06:14:51PM +0200, Jann Horn wrote:
> Since commit c9e0924e5c2b ("perf/core: open access to probes for
> CAP_PERFMON privileged process"), it is possible to create uprobes
> through perf_event_open() when the caller has CAP_PERFMON. uprobes can
> have destructive effects, while my understanding is that CAP_PERFMON
> is supposed to only let you _read_ stuff (like registers and stack
> memory) from other processes, but not modify their execution.
> 
> uprobes (at least on x86) can be destructive because they have no
> protection against poking in the middle of an instruction; basically
> as long as the kernel manages to decode the instruction bytes at the
> caller-specified offset as a relocatable instruction, a breakpoint
> instruction can be installed at that offset.
> 
> This means uprobes can be used to alter what happens in another
> process. It would probably be a good idea to go back to requiring
> CAP_SYS_ADMIN for installing uprobes, unless we can get to a point
> where the kernel can prove that the software breakpoint poke cannot
> break the target process. (Which seems harder than doing it for
> kprobe, since kprobe can at least rely on symbols to figure out where
> a function starts...)
> 
> As a small example, in one terminal:

Urrggh... x86 instruction encoding wins again. Awesome find.

Yeah, I suppose I should go queue a revert of that commit.