[PATCH 0/3] arm64: Allow the compat vdso to be disabled at runtime

Marc Zyngier
2020-07-01T16:18:34Z

The relatively recent introduction of the compat vdso on arm64 has
overlooked its interactions with some of the interesting errata
workarounds, such as ARM64_ERRATUM_1418040 (and its older 1188873
incarnation).

This erratum requires the 64bit kernel to trap 32bit accesses to the
virtual counter and emulate it. When the workaround was introduced,
the compat vdso simply wasn't a thing. Now that the patches have
landed in mainline, we trap the CVTVCT accesses from the vdso.

This can end-up in a nasty loop in the vdso, where the sequence number
changes on each trap, never stabilising, and leaving userspace in a
bit of a funny state (which is why we disable the vdso in most similar
cases). This erratum mentionned above is a bit special in the sense
that in only requires to trap AArch32 accesses, and 64bit tasks can be
left alone. Consequently, the vdso is never disabled and AArch32 tasks
are affected.

Obviously, we really want to retain the 64bit vdso in this case. To
that effect, this series offers a way to disable the 32bit view of the
vdso without impacting its 64bit counterpart, by providing a
"no-compat" vdso clock_mode, and plugging this feature into the
1418040 detection code.

I haven't put any Cc stable for now, but I believe we need to do
something for older kernels.

Marc Zyngier (3):
arm64: Introduce a way to disable the 32bit vdso
arm64: arch_timer: Allow an workaround descriptor to provide
vdso_clock_mode
arm64: timers: Disable the compat vdso for cores affected by
ARM64_WORKAROUND_1418040

arch/arm64/include/asm/arch_timer.h | 3 +++
arch/arm64/include/asm/vdso/clocksource.h | 7 +++++--
arch/arm64/include/asm/vdso/compat_gettimeofday.h | 8 +++++++-
drivers/clocksource/arm_arch_timer.c | 11 +++++++++++
4 files changed, 26 insertions(+), 3 deletions(-)

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2.27.0