Message-ID: <CACT4Y+YxmkW6opFVJFOOFd=c73gz7yFvwBBCnjMndj-jffjBCw@mail.gmail.com>
Date: Tue, 25 Feb 2025 15:07:31 +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 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.


> 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).

>     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? ;)


> 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);
> >   }
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com

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