Re: [PATCH v4 3/4] rseq: Make rseq work with protection keys

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

On 2025-02-25 09:07, Dmitry Vyukov wrote:
> On Mon, 24 Feb 2025 at 20:18, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> On 2025-02-24 08:20, Dmitry Vyukov 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.
>>
>> This is a good start, but the plan Dave and I discussed went further
>> than this. Those additions are needed:
>>
>> 1) Add validation at rseq registration that the struct rseq is indeed
>>      pkey-0 memory (return failure if not).
> 
> I don't think this is worth it for multiple reasons:
>   - a program may first register it and then assign a key, which means
> we also need to check in pkey_mprotect
>   - pkey_mprotect may be applied to rseq of another thread, so ensuring
> that will require complex code with non-trivial synchronization and
> will add considerable overhead to pkey_mprotect call
>   - a program may assign non-0 pkey but have it always accessible, such
> programs will break by the new check
>   - the misuse is already detected by rseq code, and UNIX errno-based
> reporting is not very informative and does not add much value on top
> of existing reporting
>   - this is not different from registering rseq and then unmap'ing the
> memory, checking that does not look like a good idea, and checking
> only subset of misuses is inconsistent
> 
> Based on my experience with rseq, what would be useful is reporting a
> meaningful siginfo for access errors (address/unique code) and fixing
> signal delivery. That would solve all of the above problems, and
> provide useful info for the user (not just confusing EINVAL from
> mprotect/munmap).
> 
> But I would prefer to not mix these unrelated usability improvements
> and bug fixes with this change. That's not related to this change.

I agree with your arguments. If Dave is OK with it, I'd be fine with
leaving out the pkey-0 validation on rseq registration, and eventually
bring meaningful siginfo access errors as future improvements.

So the new behavior would be that both rseq and rseq_cs are required
to be pkey-0. If they are not and their pkey is not accessible in the
current context, it would trigger a segmentation fault. Ideally we'd
want to document this somewhere in the UAPI header.

> 
> 
>> 2) The pkey-0 requirement is only for struct rseq, which we can check
>>      for at rseq registration, and happens to be the fast path. For struct
>>      rseq_cs, this is not the same tradeoff: we cannot easily check its
>>      associated pkey because the rseq_cs pointer is updated by userspace
>>      when entering a critical section. But the good news is that reading
>>      the content of struct rseq_cs is *not* a fast-path: it's only done
>>      when preempting/delivering a signal over a thread which has a
>>      non-NULL rseq_cs pointer.
> 
> rseq_cs is usually accessed on a hot path since rseq_cs pointer is not
> cleared on critical section exit (at least that's what we do).

Fair point.

> 
>>      Therefore reading the struct rseq_cs content should be done with
>>      write_permissive_pkey_val(), giving access to all pkeys.
> 
> You just asked me to redo the code to simplify it, won't this
> complicate it back again? ;)

I'm fine with the pkey-0 approach for both rseq and rseq_cs if Dave is
also OK with it.

Thanks,

Mathieu

> 
> 
>> Thanks,
>>
>> Mathieu
>>
>>>
>>> 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")
>>>
>>> ---
>>> 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
>>> ---
>>>    kernel/rseq.c | 11 +++++++++++
>>>    1 file changed, 11 insertions(+)
>>>
>>> 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);
>>>    }
>>
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com


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