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Make RDRAND a required feature for SEV-ES and -SNP #1

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Make RDRAND a required feature for SEV-ES and -SNP #1

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17 changes: 15 additions & 2 deletions arch/x86/kernel/sev-es-shared.c
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Original file line number Diff line number Diff line change
Expand Up @@ -457,9 +457,12 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
struct pt_regs *regs = ctxt->regs;
u32 cr4 = native_read_cr4();
enum es_result ret;
const u32 eax = regs->ax & 0xffffffffU;
const u32 ecx = regs->cx & 0xffffffffU;
const bool request_feature_bits = (eax == 0x1U);

ghcb_set_rax(ghcb, regs->ax & 0xffffffff);
ghcb_set_rcx(ghcb, regs->cx & 0xffffffff);
ghcb_set_rax(ghcb, eax);
ghcb_set_rcx(ghcb, ecx);

if (cr4 & X86_CR4_OSXSAVE)
/* Safe to read xcr0 */
Expand All @@ -478,6 +481,16 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
ghcb_is_valid_rdx(ghcb)))
return ES_VMM_ERROR;

/*
* Without RDRAND, KASLR relies on the TSC value for randomnization
* which is controlled by the HV either through interception or modifying
* the TSC_RATIO_MSR.
* As a hardening, RDRAND is made a minimal required feature and VM
* returns error if the HV states that RDRAND is not supported.
*/
if (request_feature_bits && !(ghcb->save.rcx & (1U << 30)))
return ES_VMM_ERROR;

regs->ax = ghcb->save.rax & 0xffffffff;
regs->bx = ghcb->save.rbx & 0xffffffff;
regs->cx = ghcb->save.rcx & 0xffffffff;
Expand Down