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Subject[RFC PATCH v1 44/50] arm64: ptr auth: Use get_random_u64 instead of _bytes
Since these are authentication keys, stored in the kernel as long
as they're important, get_random_u64 is fine. In particular,
get_random_bytes has significant per-call overhead, so five
separate calls is painful.

This ended up being a more extensive change, since the previous
code was unrolled and 10 calls to get_random_u64() seems excessive.
So the code was rearranged to have smaller object size.

Currently fields[i] = { 1 << i, 16 * i } for all i could be computed
rather than looked up, but the table seemed more future-proof.

For ptrauth_keys_switch(), the MSR instructions must be unrolled and
are much faster than get_random, so although a similar flags-based
interface is possible, it's probably not worth it.

Signed-off-by: George Spelvin <lkml@sdf.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
---
arch/arm64/include/asm/pointer_auth.h | 20 +++++----
arch/arm64/kernel/pointer_auth.c | 62 +++++++++++++++------------
2 files changed, 46 insertions(+), 36 deletions(-)

diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 7a24bad1a58b8..b7ef71362a3ae 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -30,17 +30,19 @@ struct ptrauth_keys {
struct ptrauth_key apga;
};

+static inline unsigned long ptrauth_keys_supported(void)
+{
+ return (system_supports_address_auth() ?
+ PR_PAC_APIAKEY | PR_PAC_APIBKEY |
+ PR_PAC_APDAKEY | PR_PAC_APDBKEY : 0) |
+ (system_supports_generic_auth() ? PR_PAC_APGAKEY : 0);
+}
+
+void ptrauth_keys_generate(struct ptrauth_keys *keys, unsigned long flags);
+
static inline void ptrauth_keys_init(struct ptrauth_keys *keys)
{
- if (system_supports_address_auth()) {
- get_random_bytes(&keys->apia, sizeof(keys->apia));
- get_random_bytes(&keys->apib, sizeof(keys->apib));
- get_random_bytes(&keys->apda, sizeof(keys->apda));
- get_random_bytes(&keys->apdb, sizeof(keys->apdb));
- }
-
- if (system_supports_generic_auth())
- get_random_bytes(&keys->apga, sizeof(keys->apga));
+ ptrauth_keys_generate(keys, ptrauth_keys_supported());
}

#define __ptrauth_key_install(k, v) \
diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
index c507b584259d0..1604ed246128c 100644
--- a/arch/arm64/kernel/pointer_auth.c
+++ b/arch/arm64/kernel/pointer_auth.c
@@ -7,40 +7,48 @@
#include <asm/cpufeature.h>
#include <asm/pointer_auth.h>

+/*
+ * Generating crypto-quality random numbers is expensive enough that
+ * there's no point unrolling this.
+ */
+void ptrauth_keys_generate(struct ptrauth_keys *keys, unsigned long flags)
+{
+ size_t i;
+ static const struct {
+ /*
+ * 8 bits is enough for now. Compiler will complain
+ * if/when we need more.
+ */
+ unsigned char flag, offset;
+ } fields[] = {
+ { PR_PAC_APIAKEY, offsetof(struct ptrauth_keys, apia) },
+ { PR_PAC_APIBKEY, offsetof(struct ptrauth_keys, apib) },
+ { PR_PAC_APDAKEY, offsetof(struct ptrauth_keys, apda) },
+ { PR_PAC_APDBKEY, offsetof(struct ptrauth_keys, apdb) },
+ { PR_PAC_APGAKEY, offsetof(struct ptrauth_keys, apga) }
+ };
+
+ for (i = 0; i < ARRAY_SIZE(fields); i++) {
+ if (flags & fields[i].flag) {
+ struct ptrauth_key *k = (void *)keys + fields[i].offset;
+ k->lo = get_random_u64();
+ k->hi = get_random_u64();
+ }
+ }
+}
+
int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
{
+ unsigned long supported = ptrauth_keys_supported();
struct ptrauth_keys *keys = &tsk->thread.keys_user;
- unsigned long addr_key_mask = PR_PAC_APIAKEY | PR_PAC_APIBKEY |
- PR_PAC_APDAKEY | PR_PAC_APDBKEY;
- unsigned long key_mask = addr_key_mask | PR_PAC_APGAKEY;

- if (!system_supports_address_auth() && !system_supports_generic_auth())
+ if (!supported || arg & ~supported)
return -EINVAL;

- if (!arg) {
- ptrauth_keys_init(keys);
- ptrauth_keys_switch(keys);
- return 0;
- }
-
- if (arg & ~key_mask)
- return -EINVAL;
-
- if (((arg & addr_key_mask) && !system_supports_address_auth()) ||
- ((arg & PR_PAC_APGAKEY) && !system_supports_generic_auth()))
- return -EINVAL;
-
- if (arg & PR_PAC_APIAKEY)
- get_random_bytes(&keys->apia, sizeof(keys->apia));
- if (arg & PR_PAC_APIBKEY)
- get_random_bytes(&keys->apib, sizeof(keys->apib));
- if (arg & PR_PAC_APDAKEY)
- get_random_bytes(&keys->apda, sizeof(keys->apda));
- if (arg & PR_PAC_APDBKEY)
- get_random_bytes(&keys->apdb, sizeof(keys->apdb));
- if (arg & PR_PAC_APGAKEY)
- get_random_bytes(&keys->apga, sizeof(keys->apga));
+ if (!arg)
+ arg = supported;

+ ptrauth_keys_generate(keys, arg);
ptrauth_keys_switch(keys);

return 0;
--
2.26.0
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