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From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: [RFC PATCH 4/6] x86/syscall: maximize MSR_SYSCALL_MASK
Date:   Mon, 10 May 2021 10:45:07 -0700
Message-Id: <20210510174509.3039991-5-hpa@zytor.com>
In-Reply-To: <20210510174509.3039991-1-hpa@zytor.com>
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From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

It is better to clear as many flags as possible when we do a system
call entry, as opposed to the other way around. The fewer flags we
keep, the lesser the possible interference between the kernel and user
space.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 arch/x86/kernel/cpu/common.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index a1b756c49a93..6cf697574661 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1773,10 +1773,16 @@ void syscall_init(void)
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);
 #endif
 
-	/* Flags to clear on syscall */
+	/*
+	 * Flags to clear on syscall; clear as much as possible
+	 * to minimize user space-kernel interference.
+	 */
 	wrmsrl(MSR_SYSCALL_MASK,
-	       X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF|
-	       X86_EFLAGS_IOPL|X86_EFLAGS_AC|X86_EFLAGS_NT);
+	       X86_EFLAGS_CF|X86_EFLAGS_PF|X86_EFLAGS_AF|
+	       X86_EFLAGS_ZF|X86_EFLAGS_SF|X86_EFLAGS_TF|
+	       X86_EFLAGS_IF|X86_EFLAGS_DF|X86_EFLAGS_OF|
+	       X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_RF|
+	       X86_EFLAGS_AC|X86_EFLAGS_ID);
 }
 
 #else	/* CONFIG_X86_64 */
-- 
2.31.1