path: root/Documentation/arm64
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authorCatalin Marinas <catalin.marinas@arm.com>2021-04-15 14:00:48 +0100
committerCatalin Marinas <catalin.marinas@arm.com>2021-04-15 14:00:48 +0100
commita27a8816568964fcef62a3ae5f9d2228ec1ebc68 (patch)
tree7197c94bcf5b600bb43e7aa70ba2b05a7fb82cd1 /Documentation/arm64
parentMerge branch 'for-next/mte-async-kernel-mode' into for-next/core (diff)
parentarm64: pac: Optimize kernel entry/exit key installation code paths (diff)
Merge branch 'for-next/pac-set-get-enabled-keys' into for-next/core
* for-next/pac-set-get-enabled-keys: : Introduce arm64 prctl(PR_PAC_{SET,GET}_ENABLED_KEYS). arm64: pac: Optimize kernel entry/exit key installation code paths arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS) arm64: mte: make the per-task SCTLR_EL1 field usable elsewhere
Diffstat (limited to 'Documentation/arm64')
1 files changed, 34 insertions, 0 deletions
diff --git a/Documentation/arm64/pointer-authentication.rst b/Documentation/arm64/pointer-authentication.rst
index 30b2ab06526b..f127666ea3a8 100644
--- a/Documentation/arm64/pointer-authentication.rst
+++ b/Documentation/arm64/pointer-authentication.rst
@@ -107,3 +107,37 @@ filter out the Pointer Authentication system key registers from
KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
register. Any attempt to use the Pointer Authentication instructions will
result in an UNDEFINED exception being injected into the guest.
+Enabling and disabling keys
+The prctl PR_PAC_SET_ENABLED_KEYS allows the user program to control which
+PAC keys are enabled in a particular task. It takes two arguments, the
+first being a bitmask of PR_PAC_APIAKEY, PR_PAC_APIBKEY, PR_PAC_APDAKEY
+and PR_PAC_APDBKEY specifying which keys shall be affected by this prctl,
+and the second being a bitmask of the same bits specifying whether the key
+should be enabled or disabled. For example::
+ PR_PAC_APIBKEY, 0, 0);
+disables all keys except the IB key.
+The main reason why this is useful is to enable a userspace ABI that uses PAC
+instructions to sign and authenticate function pointers and other pointers
+exposed outside of the function, while still allowing binaries conforming to
+the ABI to interoperate with legacy binaries that do not sign or authenticate
+The idea is that a dynamic loader or early startup code would issue this
+prctl very early after establishing that a process may load legacy binaries,
+but before executing any PAC instructions.
+For compatibility with previous kernel versions, processes start up with IA,
+IB, DA and DB enabled, and are reset to this state on exec(). Processes created
+via fork() and clone() inherit the key enabled state from the calling process.
+It is recommended to avoid disabling the IA key, as this has higher performance
+overhead than disabling any of the other keys.