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From: George Spelvin <lkml@sdf.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	George Spelvin <lkml@sdf.org>
Subject: [PATCH v2] arm64: ptr auth: Use get_random_u64 instead of _bytes
Date: Tue, 31 Mar 2020 15:44:12 GMT	[thread overview]
Message-ID: <202003311544.02VFiClP011630@sdf.org> (raw)

get_random_bytes() is approximately 4x the cost of two
get_random_u64() calls, because the former implements
anti-backtracking.

Because these are authentication keys, useless to an attacker
as soon as the kernel stops using them, there is no security
benefit from anti-backtracking.

Signed-off-by: George Spelvin <lkml@sdf.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
---
v2: Took out all the clever bitmap-based stuff and made a simple
    boring helper function to replace get_random_bytes(&key, 16).

 arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++-----
 arch/arm64/kernel/pointer_auth.c      | 10 +++++-----
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 7a24bad1a58b8..1a4590d05785e 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -30,17 +30,23 @@ struct ptrauth_keys {
 	struct ptrauth_key apga;
 };
 
+static inline void __ptrauth_key_init(struct ptrauth_key *key)
+{
+	key->lo = get_random_u64();
+	key->hi = get_random_u64();
+}
+
 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));
+		__ptrauth_key_init(&keys->apia);
+		__ptrauth_key_init(&keys->apib);
+		__ptrauth_key_init(&keys->apda);
+		__ptrauth_key_init(&keys->apdb);
 	}
 
 	if (system_supports_generic_auth())
-		get_random_bytes(&keys->apga, sizeof(keys->apga));
+		__ptrauth_key_init(&keys->apga);
 }
 
 #define __ptrauth_key_install(k, v)				\
diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
index c507b584259d0..05e2e3d174010 100644
--- a/arch/arm64/kernel/pointer_auth.c
+++ b/arch/arm64/kernel/pointer_auth.c
@@ -31,15 +31,15 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
 		return -EINVAL;
 
 	if (arg & PR_PAC_APIAKEY)
-		get_random_bytes(&keys->apia, sizeof(keys->apia));
+		__ptrauth_key_init(&keys->apia);
 	if (arg & PR_PAC_APIBKEY)
-		get_random_bytes(&keys->apib, sizeof(keys->apib));
+		__ptrauth_key_init(&keys->apib);
 	if (arg & PR_PAC_APDAKEY)
-		get_random_bytes(&keys->apda, sizeof(keys->apda));
+		__ptrauth_key_init(&keys->apda);
 	if (arg & PR_PAC_APDBKEY)
-		get_random_bytes(&keys->apdb, sizeof(keys->apdb));
+		__ptrauth_key_init(&keys->apdb);
 	if (arg & PR_PAC_APGAKEY)
-		get_random_bytes(&keys->apga, sizeof(keys->apga));
+		__ptrauth_key_init(&keys->apga);
 
 	ptrauth_keys_switch(keys);
 
-- 
2.26.0

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             reply	other threads:[~2020-03-31 15:45 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-31 15:44 George Spelvin [this message]
2020-03-31 16:22 ` [PATCH v2] arm64: ptr auth: Use get_random_u64 instead of _bytes Mark Rutland
2020-04-28 12:58 ` Will Deacon
2020-05-04 21:07   ` Will Deacon

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