[v4,14/38] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_{ADDRESS,GENERIC}_AUTH

Commit Message

Mark Rutland Oct. 16, 2023, 10:24 a.m. UTC

In system_supports_address_auth() and system_supports_generic_auth() we
use cpus_have_const_cap to check for ARM64_HAS_ADDRESS_AUTH and
ARM64_HAS_GENERIC_AUTH respectively, but this is not necessary and
alternative_has_cap_*() would bre preferable.

For historical reasons, cpus_have_const_cap() is more complicated than
it needs to be. Before cpucaps are finalized, it will perform a bitmap
test of the system_cpucaps bitmap, and once cpucaps are finalized it
will use an alternative branch. This used to be necessary to handle some
race conditions in the window between cpucap detection and the
subsequent patching of alternatives and static branches, where different
branches could be out-of-sync with one another (or w.r.t. alternative
sequences). Now that we use alternative branches instead of static
branches, these are all patched atomically w.r.t. one another, and there
are only a handful of cases that need special care in the window between
cpucap detection and alternative patching.

Due to the above, it would be nice to remove cpus_have_const_cap(), and
migrate callers over to alternative_has_cap_*(), cpus_have_final_cap(),
or cpus_have_cap() depending on when their requirements. This will
remove redundant instructions and improve code generation, and will make
it easier to determine how each callsite will behave before, during, and
after alternative patching.

The ARM64_HAS_ADDRESS_AUTH cpucap is a boot cpu feature which is
detected and patched early on the boot CPU before any pointer
authentication keys are enabled via their respective SCTLR_ELx.EN* bits.
Nothing which uses system_supports_address_auth() is called before the
boot alternatives are patched. Thus it is safe for
system_supports_address_auth() to use cpus_have_final_boot_cap() to
check for ARM64_HAS_ADDRESS_AUTH.

The ARM64_HAS_GENERIC_AUTH cpucap is a system feature which is detected
on all CPUs, then finalized and patched under
setup_system_capabilities(). We use system_supports_generic_auth() in a
few places:

* The pac_generic_keys_get() and pac_generic_keys_set() functions are
  only reachable from system calls once userspace is up and running. As
  cpucaps are finalzied long before userspace runs, these can safely use
  alternative_has_cap_*() or cpus_have_final_cap().

* The ptrauth_prctl_reset_keys() function is only reachable from system
  calls once userspace is up and running. As cpucaps are finalized long
  before userspace runs, this can safely use alternative_has_cap_*() or
  cpus_have_final_cap().

* The ptrauth_keys_install_user() function is used during
  context-switch. This is called prior to alternatives being applied,
  and so cannot use cpus_have_final_cap(), but as this only needs to
  switch the APGA key for userspace tasks, it's safe to use
  alternative_has_cap_*().

* The ptrauth_keys_init_user() function is used to initialize userspace
  keys, and is only reachable after system cpucaps have been finalized
  and patched. Thus this can safely use alternative_has_cap_*() or
  cpus_have_final_cap().

* The system_has_full_ptr_auth() helper function is only used by KVM
  code, which is only reachable after system cpucaps have been finalized
  and patched. Thus this can safely use alternative_has_cap_*() or
  cpus_have_final_cap().

This patch modifies system_supports_address_auth() to use
cpus_have_final_boot_cap() to check ARM64_HAS_ADDRESS_AUTH, and modifies
system_supports_generic_auth() to use alternative_has_cap_unlikely() to
check ARM64_HAS_GENERIC_AUTH. In either case this will avoid generating
code to test the system_cpucaps bitmap and should be better for all
subsequent calls at runtime. The use of cpus_have_final_boot_cap() will
make it easier to spot if code is chaanged such that these run before
the relevant cpucap is guaranteed to have been finalized.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Ard Biesheuvel <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Suzuki K Poulose <[e-mail]>
Cc: Will Deacon <[e-mail]>
---
 arch/arm64/include/asm/cpufeature.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Patch

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index a1698945916ed..dfdedbdcc1151 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -806,12 +806,12 @@  static __always_inline bool system_supports_cnp(void)
 
 static inline bool system_supports_address_auth(void)
 {
-	return cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH);
+	return cpus_have_final_boot_cap(ARM64_HAS_ADDRESS_AUTH);
 }
 
 static inline bool system_supports_generic_auth(void)
 {
-	return cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH);
+	return alternative_has_cap_unlikely(ARM64_HAS_GENERIC_AUTH);
 }
 
 static inline bool system_has_full_ptr_auth(void)