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

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Message-ID: <CACT4Y+b0cdGZwqmZSikxam+ASp9LXMuT9f8iifnmNed+PjamVg@mail.gmail.com>
Date: Fri, 18 Jul 2025 11:01:31 +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>, Stephen Röttger <sroettger@...gle.com>
Subject: Re: [PATCH v7 3/4] rseq: Make rseq work with protection keys

On Tue, 24 Jun 2025 at 11:17, 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?

Gentle ping. What needs to happen for this series to be merged?


> > > ---
> > > 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
> > >