Message-ID: <CACT4Y+Y9RodZO7T2RSJGohwjAdgBktgySCvCZ9xaPdVJafYzJw@mail.gmail.com>
Date: Tue, 25 Feb 2025 15:51:52 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: peterz@...radead.org, boqun.feng@...il.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, 
	aruna.ramakrishna@...cle.com, elver@...gle.com, 
	"Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/4] rseq: Make rseq work with protection keys

On Tue, 25 Feb 2025 at 15:28, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> On 2025-02-25 09:07, Dmitry Vyukov wrote:
> > On Mon, 24 Feb 2025 at 20:18, Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.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.

Makes sense. I will wait for Dave comments/ack before sending v6. But
to save a round-trip, does this look reasonable?

--- 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 {



> >> 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.

It should work for my current use case, at least how I currently see
it. Ways people use pkeys are pretty unique, so it's hard to
extrapolate. But there is one more possibility: when a program
switches PKEYs, it may also clear stale rseq_cs pointer from rseq.
This way rseq_cs may have non-0 keys assigned, but they are always
accessible while installed.



> Thanks,
>
> Mathieu
>
> >
> >
> >> Thanks,
> >>
> >> Mathieu
> >>
> >>>
> >>> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> >>> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >>> Cc: Peter Zijlstra <peterz@...radead.org>
> >>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> >>> Cc: Boqun Feng <boqun.feng@...il.com>
> >>> Cc: Thomas Gleixner <tglx@...utronix.de>
> >>> Cc: Ingo Molnar <mingo@...hat.com>
> >>> Cc: Borislav Petkov <bp@...en8.de>
> >>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> >>> Cc: "H. Peter Anvin" <hpa@...or.com>
> >>> Cc: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
> >>> Cc: x86@...nel.org
> >>> Cc: linux-kernel@...r.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);
> >>>    }

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