linux-kernel - Re: [PATCH v2] perf: Allow restricted kernel breakpoints on user addresses

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Date:   Wed, 1 Feb 2023 11:54:08 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Marco Elver <elver@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        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, Jann Horn <jannh@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrey Konovalov <andreyknvl@...il.com>
Subject: Re: [PATCH v2] perf: Allow restricted kernel breakpoints on user
 addresses

Hi Dmitry,

We raced to reply here, so there's more detail in my reply to Marco. I'm
providing minimal detail here, sorry for being terse! :)

On Wed, Feb 01, 2023 at 10:53:44AM +0100, Dmitry Vyukov wrote:
> On Wed, 1 Feb 2023 at 10:34, Marco Elver <elver@...gle.com> wrote:
> >
> > On Mon, 30 Jan 2023 at 11:46, Mark Rutland <mark.rutland@....com> wrote:
> > [...]
> > > > This again feels like a deficiency with access_ok(). Is there a better
> > > > primitive than access_ok(), or can we have something that gives us the
> > > > guarantee that whatever it says is "ok" is a userspace address?
> > >
> > > I don't think so, since this is contextual and temporal -- a helper can't give
> > > a single correct answert in all cases because it could change.
> >
> > That's fair, but unfortunate. Just curious: would
> > copy_from_user_nofault() reliably fail if it tries to access one of
> > those mappings but where access_ok() said "ok"?
> 
> I also wonder if these special mappings are ever accessible in a user
> task context?

No. The special mappings are actually distinct page tables from the user page
tables, so whenever userspace is executing and can issue a syscall, the user
page tables are installed.

The special mappings are only installed for transient periods within the
context of a user task. There *might* be some latent issues with work happening
in IPI context (e.g. perf user backtrace) on some architectures.

> If yes, can a racing process_vm_readv/writev mess with these special mappings?

No; those happen in task context, and cannot be invoked within the critical
section where the page tables with the special mappings are installed.

> We could use copy_from_user() to probe that the watchpoint address is
> legit. But I think the memory can be potentially PROT_NONE but still
> legit, so copy_from_user() won't work for these corner cases.

Please see my other reply; ahead-of-time checks cannot help here. An address
might be a legitimate user address and *also* transiently be a special mapping
(since the two aare in entirely separate page tables).

Thanks,
Mark.