Re: [PATCH v1] x86: Pin cr4 FSGSBASE

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Mon, May 25, 2020 at 10:28:48PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> Since there seem to be kernel modules floating around that set
> FSGSBASE incorrectly, prevent this in the CR4 pinning. Currently
> CR4 pinning just checks that bits are set, this also checks
> that the FSGSBASE bit is not set, and if it is clears it again.
> 
> Note this patch will need to be undone when the full FSGSBASE
> patches are merged. But it's a reasonable solution for v5.2+
> stable at least. Sadly the older kernels don't have the necessary
> infrastructure for this (although a simpler version of this
> could be added there too)
> 
> Cc: stable@xxxxxxxxxxxxxxx # v5.2+
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/common.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index bed0cb83fe24..1f5b7871ae9a 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -385,6 +385,11 @@ void native_write_cr4(unsigned long val)
>  		/* Warn after we've set the missing bits. */
>  		WARN_ONCE(bits_missing, "CR4 bits went missing: %lx!?\n",
>  			  bits_missing);
> +		if (val & X86_CR4_FSGSBASE) {
> +			WARN_ONCE(1, "CR4 unexpectedly set FSGSBASE!?\n");
> +			val &= ~X86_CR4_FSGSBASE;
> +			goto set_register;
> +		}
>  	}
>  }
>  EXPORT_SYMBOL(native_write_cr4);
> -- 
> 2.25.4
> 

And if this is going to be more permanent, we can separate the mask
(untested):


diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index bed0cb83fe24..ead64f7420a5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -347,6 +347,8 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
 	cr4_clear_bits(X86_CR4_UMIP);
 }
 
+static const unsigned long cr4_pinned_mask =
+	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP | X86_CR4_FSGSBASE;
 static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
 static unsigned long cr4_pinned_bits __ro_after_init;
 
@@ -371,20 +373,20 @@ EXPORT_SYMBOL(native_write_cr0);
 
 void native_write_cr4(unsigned long val)
 {
-	unsigned long bits_missing = 0;
+	unsigned long bits_changed = 0;
 
 set_register:
 	asm volatile("mov %0,%%cr4": "+r" (val), "+m" (cr4_pinned_bits));
 
 	if (static_branch_likely(&cr_pinning)) {
-		if (unlikely((val & cr4_pinned_bits) != cr4_pinned_bits)) {
-			bits_missing = ~val & cr4_pinned_bits;
-			val |= bits_missing;
+		if (unlikely((val & cr4_pinned_mask) != cr4_pinned_bits)) {
+			bits_changed = ~val & cr4_pinned_mask;
+			val = (val & ~cr4_pinned_mask) | cr4_pinned_bits;
 			goto set_register;
 		}
 		/* Warn after we've set the missing bits. */
-		WARN_ONCE(bits_missing, "CR4 bits went missing: %lx!?\n",
-			  bits_missing);
+		WARN_ONCE(bits_changed, "pinned CR4 bits changed: %lx!?\n",
+			  bits_changed);
 	}
 }
 EXPORT_SYMBOL(native_write_cr4);
@@ -396,7 +398,7 @@ void cr4_init(void)
 	if (boot_cpu_has(X86_FEATURE_PCID))
 		cr4 |= X86_CR4_PCIDE;
 	if (static_branch_likely(&cr_pinning))
-		cr4 |= cr4_pinned_bits;
+		cr4 = (cr4 & ~cr4_pinned_mask) | cr4_pinned_bits;
 
 	__write_cr4(cr4);
 
@@ -411,10 +413,7 @@ void cr4_init(void)
  */
 static void __init setup_cr_pinning(void)
 {
-	unsigned long mask;
-
-	mask = (X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP);
-	cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & mask;
+	cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask;
 	static_key_enable(&cr_pinning.key);
 }
 

-- 
Kees Cook