[v5 19/20] x86: Enable User-Mode Instruction Prevention

[Ricardo Neri]

User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
bit in %cr4.

It makes sense to enable UMIP at some point while booting, before user
spaces come up. Like SMAP and SMEP, is not critical to have it enabled
very early during boot. This is because UMIP is relevant only when there is
a userspace to be protected from. Given the similarities in relevance, it
makes sense to enable UMIP along with SMAP and SMEP.

UMIP is enabled by default. It can be disabled by adding clearcpuid=514
to the kernel parameters.

Cc: Andy Lutomirski 
Cc: Andrew Morton 
Cc: H. Peter Anvin 
Cc: Borislav Petkov 
Cc: Brian Gerst 
Cc: Chen Yucong 
Cc: Chris Metcalf 
Cc: Dave Hansen 
Cc: Fenghua Yu 
Cc: Huang Rui 
Cc: Jiri Slaby 
Cc: Jonathan Corbet 
Cc: Michael S. Tsirkin 
Cc: Paul Gortmaker 
Cc: Peter Zijlstra 
Cc: Ravi V. Shankar 
Cc: Shuah Khan 
Cc: Vlastimil Babka 
Cc: Tony Luck 
Cc: Paolo Bonzini 
Cc: Liang Z. Li 
Cc: Alexandre Julliard 
Cc: Stas Sergeev 
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Ricardo Neri 
---
 arch/x86/Kconfig             | 10 ++++++++++
 arch/x86/kernel/cpu/common.c | 16 +++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cc98d5a..b7f1226 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1735,6 +1735,16 @@ config X86_SMAP

If unsure, say Y.

+config X86_INTEL_UMIP
+	def_bool y
+	depends on CPU_SUP_INTEL
+	prompt "Intel User Mode Instruction Prevention" if EXPERT
+	---help---
+	  The User Mode Instruction Prevention (UMIP) is a security
+	  feature in newer Intel processors. If enabled, a general
+	  protection fault is issued if the instructions SGDT, SLDT,
+	  SIDT, SMSW and STR are executed in user mode.
+
 config X86_INTEL_MPX
    prompt "Intel MPX (Memory Protection Extensions)"
    def_bool n
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 58094a1..9f59eb5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -311,6 +311,19 @@ static __always_inline void setup_smap(struct cpuinfo_x86 *c)
    }
 }

+static __always_inline void setup_umip(struct cpuinfo_x86 *c)
+{
+	if (cpu_feature_enabled(X86_FEATURE_UMIP) &&
+	    cpu_has(c, X86_FEATURE_UMIP))
+		cr4_set_bits(X86_CR4_UMIP);
+	else
+		/*
+		 * Make sure UMIP is disabled in case it was enabled in a
+		 * previous boot (e.g., via kexec).
+		 */
+		cr4_clear_bits(X86_CR4_UMIP);
+}
+
 /*
  * Protection Keys are not available in 32-bit mode.
  */
@@ -1080,9 +1093,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
    /* Disable the PN if appropriate */
    squash_the_stupid_serial_number(c);

-	/* Set up SMEP/SMAP */
+	/* Set up SMEP/SMAP/UMIP */
    setup_smep(c);
    setup_smap(c);
+	setup_umip(c);

/*
     * The vendor-specific functions might have changed features.