Re: [RESEND][PATCH 3/3] rseq: Ensure SIGBUS delivered on memory failure

Peter Zijlstra
2024-08-06T07:52:00Z

On Mon, Aug 05, 2024 at 09:37:48PM -0700, Kees Cook wrote:
> On Tue, Jul 23, 2024 at 04:47:52PM +0200, Andrew Zaborowski wrote:
> > Uncorrected memory errors for user pages are signaled to processes
> > using SIGBUS or, if the error happens in a syscall, an error retval
> > from the syscall. The SIGBUS is documented in
> > Documentation/mm/hwpoison.rst#failure-recovery-modes
> >
> > Once a user task sets t->rseq in the rseq() syscall, if the kernel
> > cannot access the memory pointed to by t->rseq->rseq_cs, that initial
> > rseq() and all future syscalls should return an error so understandably
> > the code just kills the task.
> >
> > To ensure that SIGBUS is used set the new t->kill_on_efault flag and
> > run queued task work on rseq_get_rseq_cs() errors to give memory_failure
> > the chance to run.
> >
> > Note: the rseq checks run inside resume_user_mode_work() so whenever
> > _TIF_NOTIFY_RESUME is set. They do not run on every syscall exit so
> > I'm not concerned that these extra flag operations are in a hot path,
> > except with CONFIG_DEBUG_RSEQ.
> >
> > Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
> > ---
> > kernel/rseq.c | 25 +++++++++++++++++++++----
>
> Can an rseq maintainer please review this? I can carry it via the execve
> tree with the related patches...

*sigh*,.. because get_maintainers just doesn't work or something?

Anyway, I'm confused by the signal code (as always), why isn't the
task_work_run() in get_signal() sufficient?

At some point we're going to run into trouble with sprinkling
task_work_run() around willy nilly :/

>
> > 1 file changed, 21 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/rseq.c b/kernel/rseq.c
> > index 9de6e35fe..c5809cd13 100644
> > --- a/kernel/rseq.c
> > +++ b/kernel/rseq.c
> > @@ -13,6 +13,7 @@
> > #include <linux/syscalls.h>
> > #include <linux/rseq.h>
> > #include <linux/types.h>
> > +#include <linux/task_work.h>
> > #include <asm/ptrace.h>
> >
> > #define CREATE_TRACE_POINTS
> > @@ -320,6 +321,8 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> > if (unlikely(t->flags & PF_EXITING))
> > return;
> >
> > + t->kill_on_efault = true;
> > +
> > /*
> > * 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
> > @@ -330,13 +333,18 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> > if (unlikely(ret < 0))
> > goto error;
> > }
> > - if (unlikely(rseq_update_cpu_node_id(t)))
> > - goto error;
> > - return;
> > + if (likely(!rseq_update_cpu_node_id(t)))
> > + goto out;
> >
> > error:
> > + /* Allow task work to override signr */
> > + task_work_run();
> > +
> > sig = ksig ? ksig->sig : 0;
> > force_sigsegv(sig);
> > +
> > +out:
> > + t->kill_on_efault = false;
> > }
> >
> > #ifdef CONFIG_DEBUG_RSEQ
> > @@ -353,8 +361,17 @@ void rseq_syscall(struct pt_regs *regs)
> >
> > if (!t->rseq)
> > return;
> > - if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
> > +
> > + t->kill_on_efault = true;
> > +
> > + if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) {
> > + /* Allow task work to override signr */
> > + task_work_run();
> > +
> > force_sig(SIGSEGV);
> > + }
> > +
> > + t->kill_on_efault = false;
> > }
> >
> > #endif
> > --
> > 2.43.0
> >
>
> --
> Kees Cook