Re: [PATCH v6 3/4] rseq: Make rseq work with protection keys

[Dmitry Vyukov]

On Mon, 10 Mar 2025 at 15:31, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> On 2025-03-08 05:02, Dmitry Vyukov wrote:
> > On Thu, 27 Feb 2025 at 15:03, 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>
> >> Cc: 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")
> >
> > Any remaining concerns with this series?
> >
> > What tree should it go into?
>
> Usually the rseq bits go through the -tip tree.

Thomas, Ingo, can you please take this to -tip tree? Or who would that be?


> Thanks,
>
> Mathieu
>
> >
> >> ---
> >> 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 2cb16091ec0ae..9d9c976d3b78c 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>
> >> @@ -402,11 +403,19 @@ 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
> >> @@ -419,9 +428,11 @@ 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);
> >>   }
> >> --
> >> 2.48.1.658.g4767266eb4-goog
> >>
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com