linux-kernel - Re: [PATCH] perf: Allow restricted kernel breakpoints on user addresses

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Message-ID: <YxevqB2OpJ9BLE+s@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Sep 2022 22:38:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, Dmitry Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] perf: Allow restricted kernel breakpoints on user
 addresses

On Fri, Sep 02, 2022 at 12:00:57PM +0200, Marco Elver wrote:
> Allow the creation of restricted breakpoint perf events that also fire
> in the kernel (!exclude_kernel), if:
> 
>   1. No sample information is requested; samples may contain IPs,
>      registers, or other information that may disclose kernel addresses.
> 
>   2. The breakpoint (viz. data watchpoint) is on a user address.
> 
> The rules constrain the allowable perf events such that no sensitive
> kernel information can be disclosed.
> 
> Despite no explicit kernel information disclosure, the following
> questions may need answers:
> 
>  1. Is obtaining information that the kernel accessed a particular
>     user's known memory location revealing new information?
>     Given the kernel's user space ABI, there should be no "surprise
>     accesses" to user space memory in the first place.
> 
>  2. Does causing breakpoints on user memory accesses by the kernel
>     potentially impact timing in a sensitive way?
>     Since hardware breakpoints trigger regardless of the state of
>     perf_event_attr::exclude_kernel, but are filtered in the perf
>     subsystem, this possibility already exists independent of the
>     proposed change.
> 

Changelog forgot to tell us why you want this :-)

I don't see any immediate concerns, but it's late so who knows..