linux-kernel - [RFC v2 PATCH 4/7] x86/syscall: maximize MSR_SYSCALL

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Date:   Mon, 10 May 2021 11:53:13 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC v2 PATCH 4/7] x86/syscall: maximize MSR_SYSCALL_MASK

From: "H. Peter Anvin (Intel)" <hpa@...or.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.

The flags changed are:

CF, PF, AF, ZF, SF, OF: these are arithmetic flags which affect
branches, possibly speculatively. They should be cleared for the same
reasons we now clear all GPRs on entry.

RF: suppresses a code breakpoint on the subsequent instruction. It is
probably impossible to enter the kernel with RF set, but if it is
somehow not, it would break a kernel debugger setting a breakpoint on
the entry point. Either way, user space should not be able to control
kernel behavior here.

ID: this flag has no direct effect (it is a scratch bit only.)
However, there is no reason to retain the user space value in the
kernel, and the standard should be to clear unless needed, not the
other way around.

Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.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