arm64/gcs: Implement shadow stack prctl() interface

Implement the architecture neutral prctl() interface for setting the
shadow stack status, this supports setting and reading the current GCS
configuration for the current thread.

Userspace can enable basic GCS functionality and additionally also
support for GCS pushes and arbitrary GCS stores.  It is expected that
this prctl() will be called very early in application startup, for
example by the dynamic linker, and not subsequently adjusted during
normal operation.  Users should carefully note that after enabling GCS
for a thread GCS will become active with no call stack so it is not
normally possible to return from the function that invoked the prctl().

State is stored per thread, enabling GCS for a thread causes a GCS to be
allocated for that thread.

Userspace may lock the current GCS configuration by specifying
PR_SHADOW_STACK_ENABLE_LOCK, this prevents any further changes to the
GCS configuration via any means.

If GCS is not being enabled then all flags other than _LOCK are ignored,
it is not possible to enable stores or pops without enabling GCS.

When disabling the GCS we do not free the allocated stack, this allows
for inspection of the GCS after disabling as part of fault reporting.
Since it is not an expected use case and since it presents some
complications in determining what to do with previously initialsed data
on the GCS attempts to reenable GCS after this are rejected.  This can
be revisted if a use case arises.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-23-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 5260788247d8..37fefdc3d3a3 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -187,6 +187,7 @@ struct thread_struct {
 	u64			por_el0;
 #ifdef CONFIG_ARM64_GCS
 	unsigned int		gcs_el0_mode;
+	unsigned int		gcs_el0_locked;
 	u64			gcspr_el0;
 	u64			gcs_base;
 	u64			gcs_size;