uprobes are destructive but exposed by perf under CAP_PERFMON

[Peter Zijlstra]

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.