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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-07-18 05:01, Dmitry Vyukov wrote:
> On Tue, 24 Jun 2025 at 11:17, Dmitry Vyukov <dvyukov@google.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@google.com>
>>>> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>> Cc: Peter Zijlstra <peterz@infradead.org>
>>>> Cc: "Paul E. McKenney" <paulmck@kernel.org>
>>>> Cc: Boqun Feng <boqun.feng@gmail.com>
>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>> Cc: Ingo Molnar <mingo@redhat.com>
>>>> Cc: Borislav Petkov <bp@alien8.de>
>>>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>>>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>>>> Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
>>>> Cc: x86@kernel.org
>>>> Cc: linux-kernel@vger.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?

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