Re: [PATCH 4/4] riscv: Enable perf counters user access only through perf

[Andrew Jones]

On Wed, Apr 26, 2023 at 03:17:01PM +0200, Alexandre Ghiti wrote:
> On Wed, Apr 26, 2023 at 2:57 PM Andrew Jones <ajones@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Apr 13, 2023 at 06:17:25PM +0200, Alexandre Ghiti wrote:
> > > We used to unconditionnally expose the cycle and instret csrs to
> > > userspace, which gives rise to security concerns.
> > >
> > > So only allow access to hw counters from userspace through the perf
> > > framework which will handle context switchs, per-task events...etc. But
> > > as we cannot break userspace, we give the user the choice to go back to
> > > the previous behaviour by setting the sysctl perf_user_access.
> > >
> > > We also introduce a means to directly map the hardware counters to
> > > userspace, thus avoiding the need for syscalls whenever an application
> > > wants to access counters values.
> > >
> > > Note that arch_perf_update_userpage is a copy of arm64 code.
> > >
> > > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > > ---
> > >  Documentation/admin-guide/sysctl/kernel.rst |  23 +++-
> > >  arch/riscv/include/asm/perf_event.h         |   3 +
> > >  arch/riscv/kernel/Makefile                  |   2 +-
> > >  arch/riscv/kernel/perf_event.c              |  65 +++++++++++
> > >  drivers/perf/riscv_pmu.c                    |  42 ++++++++
> > >  drivers/perf/riscv_pmu_legacy.c             |  17 +++
> > >  drivers/perf/riscv_pmu_sbi.c                | 113 ++++++++++++++++++--
> > >  include/linux/perf/riscv_pmu.h              |   3 +
> > >  tools/lib/perf/mmap.c                       |  65 +++++++++++
> > >  9 files changed, 322 insertions(+), 11 deletions(-)
> > >  create mode 100644 arch/riscv/kernel/perf_event.c
> > >
> > > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > > index 4b7bfea28cd7..02b2a40a3647 100644
> > > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > > @@ -941,16 +941,31 @@ enabled, otherwise writing to this file will return ``-EBUSY``.
> > >  The default value is 8.
> > >
> > >
> > > -perf_user_access (arm64 only)
> > > -=================================
> > > +perf_user_access (arm64 and riscv only)
> > > +=======================================
> > > +
> > > +Controls user space access for reading perf event counters.
> > >
> > > -Controls user space access for reading perf event counters. When set to 1,
> > > -user space can read performance monitor counter registers directly.
> > > +arm64
> > > +=====
> > >
> > >  The default value is 0 (access disabled).
> > > +When set to 1, user space can read performance monitor counter registers
> > > +directly.
> > >
> > >  See Documentation/arm64/perf.rst for more information.
> > >
> > > +riscv
> > > +=====
> > > +
> > > +When set to 0, user access is disabled.
> > > +
> > > +When set to 1, user space can read performance monitor counter registers
> > > +directly only through perf, any direct access without perf intervention will
> > > +trigger an illegal instruction.
> > > +
> > > +The default value is 2, it enables the legacy mode, that is user space has
> > > +direct access to cycle, time and insret CSRs only.
> >
> > I think this default value should be a Kconfig symbol, allowing kernels to
> > be built with a secure default.
> 
> Actually I was more in favor of having the default to 1 (ie the secure
> option) and let the distros deal with the legacy mode (via a sysctl
> parameter on the command line) as long as user-space has not been
> fixed: does that make sense?

Yes, I'd prefer that too. I assumed the default was 2 in this patch
because we couldn't set it to 1 for some reason.

Thanks,
drew