linux-kernel - Re: [PATCH v7 3/4] rseq: Make rseq work with protection keys

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Message-ID: <CACT4Y+anDdNU9rh1xsDRs7vZRfXbbvjFS3RRBu1zVejrp11Scw@mail.gmail.com>
Date: Tue, 24 Jun 2025 11:17:02 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: mathieu.desnoyers@...icios.com, peterz@...radead.org, boqun.feng@...il.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, hpa@...or.com, aruna.ramakrishna@...cle.com, 
	elver@...gle.com
Cc: "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org, 
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v7 3/4] rseq: Make rseq work with protection keys

On Wed, 21 May 2025 at 10:59, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Wed, 21 May 2025 at 10:52, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >
> > If an application registers rseq, and ever switches to another pkey
> > protection (such that the rseq becomes inaccessible), then any
> > context switch will cause failure in __rseq_handle_notify_resume()
> > attempting to read/write struct rseq and/or rseq_cs. Since context
> > switches are asynchronous and are outside of the application control
> > (not part of the restricted code scope), temporarily switch to
> > pkey value that allows access to the 0 (default) PKEY.
> >
> > Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
> > Cc: x86@...nel.org
> > Cc: linux-kernel@...r.kernel.org
> > Fixes: d7822b1e24f2 ("rseq: Introduce restartable sequences system call")
>
> Dave, can you please ack this patch? Ingo said he was waiting for your
> review before taking this to -tip.

Are there any remaining concerns with this series? If not, Thomas,
Ingo, can you please take this to -tip tree?

> > ---
> > Changes in v7:
> >  - Added Mathieu's Reviewed-by
> >
> > Changes in v6:
> >  - Added a comment to struct rseq with MPK rules
> >
> > Changes in v4:
> >  - Added Fixes tag
> >
> > Changes in v3:
> >  - simplify control flow to always enable access to 0 pkey
> >
> > Changes in v2:
> >  - fixed typos and reworded the comment
> > ---
> >  include/uapi/linux/rseq.h |  4 ++++
> >  kernel/rseq.c             | 11 +++++++++++
> >  2 files changed, 15 insertions(+)
> >
> > diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> > index c233aae5eac90..019fd248cf749 100644
> > --- a/include/uapi/linux/rseq.h
> > +++ b/include/uapi/linux/rseq.h
> > @@ -58,6 +58,10 @@ struct rseq_cs {
> >   * contained within a single cache-line.
> >   *
> >   * A single struct rseq per thread is allowed.
> > + *
> > + * If struct rseq or struct rseq_cs is used with Memory Protection Keys,
> > + * then the assigned pkey should either be accessible whenever these structs
> > + * are registered/installed, or they should be protected with pkey 0.
> >   */
> >  struct rseq {
> >         /*
> > diff --git a/kernel/rseq.c b/kernel/rseq.c
> > index b7a1ec327e811..88fc8cb789b3b 100644
> > --- a/kernel/rseq.c
> > +++ b/kernel/rseq.c
> > @@ -10,6 +10,7 @@
> >
> >  #include <linux/sched.h>
> >  #include <linux/uaccess.h>
> > +#include <linux/pkeys.h>
> >  #include <linux/syscalls.h>
> >  #include <linux/rseq.h>
> >  #include <linux/types.h>
> > @@ -424,11 +425,19 @@ static int rseq_ip_fixup(struct pt_regs *regs)
> >  void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> >  {
> >         struct task_struct *t = current;
> > +       pkey_reg_t saved_pkey;
> >         int ret, sig;
> >
> >         if (unlikely(t->flags & PF_EXITING))
> >                 return;
> >
> > +       /*
> > +        * Enable access to the default (0) pkey in case the thread has
> > +        * currently disabled access to it and struct rseq/rseq_cs has
> > +        * 0 pkey assigned (the only supported value for now).
> > +        */
> > +       saved_pkey = enable_zero_pkey_val();
> > +
> >         /*
> >          * regs is NULL if and only if the caller is in a syscall path.  Skip
> >          * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
> > @@ -441,9 +450,11 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> >         }
> >         if (unlikely(rseq_update_cpu_node_id(t)))
> >                 goto error;
> > +       write_pkey_val(saved_pkey);
> >         return;
> >
> >  error:
> > +       write_pkey_val(saved_pkey);
> >         sig = ksig ? ksig->sig : 0;
> >         force_sigsegv(sig);
> >  }
> > --
> > 2.49.0.1143.g0be31eac6b-goog
> >