arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS)

This change introduces a prctl that allows the user program to control
which PAC keys are enabled in a particular task. The main reason
why this is useful is to enable a userspace ABI that uses PAC 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 pointers.

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.

This change adds a small amount of overhead to kernel entry and exit
due to additional required instruction sequences.

On a DragonBoard 845c (Cortex-A75) with the powersave governor, the
overhead of similar instruction sequences was measured as 4.9ns when
simulating the common case where IA is left enabled, or 43.7ns when
simulating the uncommon case where IA is disabled. These numbers can
be seen as the worst case scenario, since in more realistic scenarios
a better performing governor would be used and a newer chip would be
used that would support PAC unlike Cortex-A75 and would be expected
to be faster than Cortex-A75.

On an Apple M1 under a hypervisor, the overhead of the entry/exit
instruction sequences introduced by this patch was measured as 0.3ns
in the case where IA is left enabled, and 33.0ns in the case where
IA is disabled.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Link: https://linux-review.googlesource.com/id/Ibc41a5e6a76b275efbaa126b31119dc197b927a5
Link: https://lore.kernel.org/r/d6609065f8f40397a4124654eb68c9f490b4d477.1616123271.git.pcc@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 170f42fd6101..eb2f73939b7b 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -909,6 +909,38 @@ static int pac_mask_get(struct task_struct *target,
 	return membuf_write(&to, &uregs, sizeof(uregs));
 }
 
+static int pac_enabled_keys_get(struct task_struct *target,
+				const struct user_regset *regset,
+				struct membuf to)
+{
+	long enabled_keys = ptrauth_get_enabled_keys(target);
+
+	if (IS_ERR_VALUE(enabled_keys))
+		return enabled_keys;
+
+	return membuf_write(&to, &enabled_keys, sizeof(enabled_keys));
+}
+
+static int pac_enabled_keys_set(struct task_struct *target,
+				const struct user_regset *regset,
+				unsigned int pos, unsigned int count,
+				const void *kbuf, const void __user *ubuf)
+{
+	int ret;
+	long enabled_keys = ptrauth_get_enabled_keys(target);
+
+	if (IS_ERR_VALUE(enabled_keys))
+		return enabled_keys;
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &enabled_keys, 0,
+				 sizeof(long));
+	if (ret)
+		return ret;
+
+	return ptrauth_set_enabled_keys(target, PR_PAC_ENABLED_KEYS_MASK,
+					enabled_keys);
+}
+
 #ifdef CONFIG_CHECKPOINT_RESTORE
 static __uint128_t pac_key_to_user(const struct ptrauth_key *key)
 {
@@ -1074,6 +1106,7 @@ enum aarch64_regset {
 #endif
 #ifdef CONFIG_ARM64_PTR_AUTH
 	REGSET_PAC_MASK,
+	REGSET_PAC_ENABLED_KEYS,
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	REGSET_PACA_KEYS,
 	REGSET_PACG_KEYS,
@@ -1160,6 +1193,14 @@ static const struct user_regset aarch64_regsets[] = {
 		.regset_get = pac_mask_get,
 		/* this cannot be set dynamically */
 	},
+	[REGSET_PAC_ENABLED_KEYS] = {
+		.core_note_type = NT_ARM_PAC_ENABLED_KEYS,
+		.n = 1,
+		.size = sizeof(long),
+		.align = sizeof(long),
+		.regset_get = pac_enabled_keys_get,
+		.set = pac_enabled_keys_set,
+	},
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	[REGSET_PACA_KEYS] = {
 		.core_note_type = NT_ARM_PACA_KEYS,