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

[Mathieu Desnoyers]

On 2025-07-18 05:01, Dmitry Vyukov wrote:
> On Tue, 24 Jun 2025 at 11:17, Dmitry Vyukov <dvyukov@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > > > Cc: Aruna Ramakrishna <aruna.ramakrishna@xxxxxxxxxx>
> > > > Cc: x86@xxxxxxxxxx
> > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > 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?

This series looks OK from my perspective. I think the last piece that
was missing was to get a review from Dave Hansen.

Dave ?

Thanks,

Mathieu

> > > > ---
> > > > 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


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com