linux-kernel - [PATCH] remove AND operation in choose_random_kstack

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Date: Mon, 17 Jun 2024 13:37:21 +0000
From: Yuntao Liu <liuyuntao12@...wei.com>
To: <x86@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<linux-hardening@...r.kernel.org>
CC: <catalin.marinas@....com>, <will@...nel.org>, <hca@...ux.ibm.com>,
	<gor@...ux.ibm.com>, <agordeev@...ux.ibm.com>, <borntraeger@...ux.ibm.com>,
	<svens@...ux.ibm.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>, <hpa@...or.com>,
	<kees@...nel.org>, <gustavoars@...nel.org>, <arnd@...db.de>,
	<leobras@...hat.com>, <broonie@...nel.org>, <mark.rutland@....com>,
	<imbrenda@...ux.ibm.com>, <pawan.kumar.gupta@...ux.intel.com>,
	<liuyuntao12@...wei.com>
Subject: [PATCH] remove AND operation in choose_random_kstack_offset()

Since the offset would be bitwise ANDed with 0x3FF in
add_random_kstack_offset(), so just remove AND operation here.

Signed-off-by: Yuntao Liu <liuyuntao12@...wei.com>
---
 arch/arm64/kernel/syscall.c          | 2 +-
 arch/s390/include/asm/entry-common.h | 2 +-
 arch/x86/include/asm/entry-common.h  | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index ad198262b981..43f555f7cd2d 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -63,7 +63,7 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 	 *
 	 * The resulting 5 bits of entropy is seen in SP[8:4].
 	 */
-	choose_random_kstack_offset(get_random_u16() & 0x1FF);
+	choose_random_kstack_offset(get_random_u16());
 }
 
 static inline bool has_syscall_work(unsigned long flags)
diff --git a/arch/s390/include/asm/entry-common.h b/arch/s390/include/asm/entry-common.h
index 7f5004065e8a..35555c944630 100644
--- a/arch/s390/include/asm/entry-common.h
+++ b/arch/s390/include/asm/entry-common.h
@@ -54,7 +54,7 @@ static __always_inline void arch_exit_to_user_mode(void)
 static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 						  unsigned long ti_work)
 {
-	choose_random_kstack_offset(get_tod_clock_fast() & 0xff);
+	choose_random_kstack_offset(get_tod_clock_fast());
 }
 
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 7e523bb3d2d3..b28a307f2014 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -85,7 +85,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	 * Therefore, final stack offset entropy will be 5 (x86_64) or
 	 * 6 (ia32) bits.
 	 */
-	choose_random_kstack_offset(rdtsc() & 0xFF);
+	choose_random_kstack_offset(rdtsc());
 }
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
 
-- 
2.34.1