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[PATCH] x86/AMD: disable RDSEED on Fam17 model 47 stepping 0


  • To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 24 Jul 2025 14:56:50 +0200
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 24 Jul 2025 12:57:01 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

This particular variant has an error that causes RDSEED to always return
0xffffffff, while RDRAND works correctly.

Inspired by Linux commit 5b937a1ed64ebeba8876e398110a5790ad77407c
("x86/rdrand: Disable RDSEED on AMD Cyan Skillfish").

Like for RDRAND, permit a command line override to be used to keep
RDSEED enabled.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Considering how it is described, I didn't think probing RDSEED (like we
do for RDRAND) would be necessary.

Am I going too far in also updating cpuidmask_defaults here, or is us
not doing so for the RDRAND disabling actually an oversight?

Using warning_add() may not be quite appropriate, as we don't really
mean the admin to possibly override this with "cpuid=rdseed" (that's
only a last resort, in case the issue is yet more limited in scope). But
mere printk() would feel like hiding the information in the middle of
lots of other output.

--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -123,6 +123,10 @@ static void __init cf_check _parse_xen_c
     else if ( feat == X86_FEATURE_RDRAND &&
               (cpuid_ecx(1) & cpufeat_mask(X86_FEATURE_RDRAND)) )
         setup_force_cpu_cap(X86_FEATURE_RDRAND);
+    else if ( feat == X86_FEATURE_RDSEED &&
+              cpuid_eax(0) >= 7 &&
+              (cpuid_count_ebx(7, 0) & cpufeat_mask(X86_FEATURE_RDSEED)) )
+        setup_force_cpu_cap(X86_FEATURE_RDSEED);
 }
 
 static int __init cf_check parse_xen_cpuid(const char *s)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -1220,6 +1220,24 @@ static void cf_check init_amd(struct cpu
                }
                break;
 
+       case 0x17:
+               /*
+                * Fam17 model 47 stepping 0 has an error that causes RDSEED to
+                * always return 0xffffffff (while RDRAND works correctly).
+                */
+               if (c == &boot_cpu_data &&
+                   c->family == 0x47 && c->stepping == 0 &&
+                   cpu_has(c, X86_FEATURE_RDSEED) &&
+                   !is_forced_cpu_cap(X86_FEATURE_RDSEED)) {
+                       static const char __initconst text[] =
+                               "RDSEED is unreliable on this hardware; 
disabling its exposure\n";
+
+                       setup_clear_cpu_cap(X86_FEATURE_RDSEED);
+                       cpuidmask_defaults._7ab0 &= 
~cpufeat_mask(X86_FEATURE_RDSEED);
+                       warning_add(text);
+               }
+               break;
+
        case 0x19:
                /*
                 * Zen3 (Fam19h model < 0x10) parts are not susceptible to



 


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