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

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

On 2025-03-08 05:02, Dmitry Vyukov wrote:
> On Thu, 27 Feb 2025 at 15:03, 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>
>> Cc: 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")
> 
> Any remaining concerns with this series?
> 
> What tree should it go into?

Usually the rseq bits go through the -tip tree.

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