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

[Dmitry Vyukov]

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?

> ---
> 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
>