x86/tdx: Make RDRAND/RDSEED loop forever

The hardware random generator behind RDRAND/RDSEED has limited
bandwidth and can be in theory starved for some time by other cores.
That's an potential attack against a TDX guest's random number
generation. The RDRAND/RDSEED users generally fall back to some other
method if the default retry loop (10 retries) fails, but these other
methods are usually guessable by a malicious host (like RDTSC or
interrupt timing or some other methods), which might help the host with
guessing

Just make them loop forever inside TDX until a valid random number
is retrieved.

While that's a potential DOS, a malicious host has already other
ways to DOS. It cannot be used for a DOS from other guests, because
those will be eventually interrupted, which allows this guest to make
progress.

Note this only affects the kernel users.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

arch/x86/include/asm/archrandom.h

@@ -10,13 +10,24 @@
 #ifndef ASM_X86_ARCHRANDOM_H
 #define ASM_X86_ARCHRANDOM_H
 
+#include <linux/protected_guest.h>
 #include <asm/processor.h>
 #include <asm/cpufeature.h>
 
 #define RDRAND_RETRY_LOOPS	10
 
 /* Unconditional execution of RDRAND and RDSEED */
 
+static inline bool rd_loop(void)
+{
+#if defined(BOOT_COMPRESSED_MISC_H) && defined(CONFIG_INTEL_TDX_GUEST)
+	/* In decompression code */
+	return early_is_tdx_guest();
+#else
+	return prot_guest_has(PATTR_GUEST_RAND_LOOP);
+#endif
+}
+
 static inline bool __must_check rdrand_long(unsigned long *v)
 {
 	bool ok;
@@ -27,7 +38,7 @@ static inline bool __must_check rdrand_long(unsigned long *v)
 			     : CC_OUT(c) (ok), [out] "=r" (*v));
 		if (ok)
 			return true;
-	} while (--retry);
+	} while (--retry || rd_loop());
 	return false;
 }
 
@@ -41,26 +52,34 @@ static inline bool __must_check rdrand_int(unsigned int *v)
 			     : CC_OUT(c) (ok), [out] "=r" (*v));
 		if (ok)
 			return true;
-	} while (--retry);
+	} while (--retry || rd_loop());
 	return false;
 }
 
 static inline bool __must_check rdseed_long(unsigned long *v)
 {
 	bool ok;
-	asm volatile("rdseed %[out]"
-		     CC_SET(c)
-		     : CC_OUT(c) (ok), [out] "=r" (*v));
-	return ok;
+	do {
+		asm volatile("rdseed %[out]"
+			     CC_SET(c)
+			     : CC_OUT(c) (ok), [out] "=r" (*v));
+		if (ok)
+			return ok;
+	} while (rd_loop());
+	return false;
 }
 
 static inline bool __must_check rdseed_int(unsigned int *v)
 {
 	bool ok;
-	asm volatile("rdseed %[out]"
-		     CC_SET(c)
-		     : CC_OUT(c) (ok), [out] "=r" (*v));
-	return ok;
+	do {
+		asm volatile("rdseed %[out]"
+			     CC_SET(c)
+			     : CC_OUT(c) (ok), [out] "=r" (*v));
+		if (ok)
+			return ok;
+	} while (rd_loop());
+	return false;
 }
 
 /*

arch/x86/kernel/tdx.c

@@ -152,6 +152,7 @@ bool tdx_prot_guest_has(unsigned long flag)
 	case PATTR_MEM_ENCRYPT:
 	case PATTR_GUEST_SECURE_TIME:
 	case PATTR_GUEST_CPUID_FILTER:
+	case PATTR_GUEST_RAND_LOOP:
 		return tdx_guest_enabled;
 	case PATTR_GUEST_DRIVER_FILTER:
 		return tdg_filter_enabled() && tdx_guest_enabled;

include/linux/protected_guest.h

@@ -21,6 +21,7 @@
 #define PATTR_GUEST_DRIVER_FILTER	6	/* Guest Driver filter */
 #define PATTR_GUEST_SECURE_TIME		7	/* Guest Secure Time */
 #define PATTR_GUEST_CPUID_FILTER	8	/* Guest CPUID Filter */
+#define PATTR_GUEST_RAND_LOOP		9	/* Hardware RNG must succeed */
 
 /* 0x800 - 0x8ff reserved for AMD */
 #define PATTR_SME			0x800