[RFC PATCH 4/6] x86/syscall: maximize MSR_SYSCALL_MASK

From: H. Peter Anvin
Date: Mon May 10 2021 - 13:45:48 EST


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

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@xxxxxxxxx>
---
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