[PATCH RESEND v9 07/36] x86/cpu: Add X86_CR4_FRED macro

From: Xin Li
Date: Tue Aug 01 2023 - 05:03:59 EST

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

Add X86_CR4_FRED macro for the FRED bit in %cr4. This bit should be a
pinned bit, not to be changed after initialization.

CR4 macros are defined in arch/x86/include/uapi/asm/processor-flags.h,
which is uapi, and thus cannot depend on CONFIG_X86_64.

Using _BITUL() causes build errors on 32 bits, and there is no
guarantee that user space applications (e.g. something like Qemu)
might not want to use this declaration even when building for i386 or

However, %cr4 is a machine word (unsigned long), so to avoid build
warnings on 32 bits, explicitly cast the value to unsigned long,
truncating upper 32 bits.

The other alternative would be to use CONFIG_X86_64 around the
definition of cr4_pinned_mask. It is probably not desirable to make
cr4_pinned_mask non-const.

Another option, which may be preferable, to be honest: explicitly
enumerate the CR4 bits which *may* be changed (a whitelist), instead
of the ones that may not. That would be a separate, pre-FRED, patch,
and would automatically resolve this problem as a side effect.

The following flags probably should have been in this set all along,
as they are all controls affecting the kernel runtime environment as
opposed to user space:

X86_CR4_DE, X86_CR4_PAE, X86_CR4_PSE, X86_CR4_MCE, X86_CR4_PGE,

Possibly X86_CR4_VMXE as well, which seems harmless even if KVM is
not loaded; X86_CR4_PKE can be fixed as long as the PKE configuration
registers are at least initialized to disabled.

It is relatively simple to do an audit of which flags are allowed to
be modified at runtime and whitelist only those. There is no reason
why we should allow bits in CR4 to be toggled by default.

Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
arch/x86/include/uapi/asm/processor-flags.h | 2 ++
arch/x86/kernel/cpu/common.c | 5 +++--
2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/uapi/asm/processor-flags.h b/arch/x86/include/uapi/asm/processor-flags.h
index d898432947ff..ce08c2ca70b5 100644
--- a/arch/x86/include/uapi/asm/processor-flags.h
+++ b/arch/x86/include/uapi/asm/processor-flags.h
@@ -138,6 +138,8 @@
#define X86_CR4_CET _BITUL(X86_CR4_CET_BIT)
#define X86_CR4_LAM_SUP_BIT 28 /* LAM for supervisor pointers */
+#define X86_CR4_FRED_BIT 32 /* enable FRED kernel entry */
+#define X86_CR4_FRED _BITULL(X86_CR4_FRED_BIT)

* x86-64 Task Priority Register, CR8
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0ba1067f4e5f..331b06d19f7f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -402,8 +402,9 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)

/* These bits should not change their value after CPU init is finished. */
static const unsigned long cr4_pinned_mask =
- X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
+ (unsigned long)
+ (X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
+ X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED);
static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
static unsigned long cr4_pinned_bits __ro_after_init;