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KVM: x86: Disallow EFER.LME and EFER.LMA if long mode is not supported

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Remove EFER.LME and EFER.LMA from EFER reserved bits only if long mode
is actually supported. KVM does check long-mode support before allowing
the bits for guest writes and userspace writes through KVM_SET_SREGS*
(in __kvm_valid_efer()), but userspace writes through KVM_SET_MSRS only
check reserved bits.

In practice, this doesn't really matter. The true motiviation is getting
rid of the #ifdeffery when initializing efer_reserved_bits.

Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260713181020.2735367-3-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Yosry Ahmed 2026-07-13 18:10:17 +00:00 committed by Sean Christopherson
parent 6ccc19d4c1
commit e62392bf39
2 changed files with 4 additions and 9 deletions
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10
arch/x86/kvm/msrs.c
View File

@ -19,16 +19,8 @@ bool __read_mostly report_ignored_msrs = true;
module_param(report_ignored_msrs, bool, 0644);
EXPORT_SYMBOL_FOR_KVM_INTERNAL(report_ignored_msrs);
/* EFER defaults:
* - enable syscall per default because its emulated by KVM
* - enable LME and LMA per default on 64 bit KVM
*/
#ifdef CONFIG_X86_64
static
u64 __read_mostly efer_reserved_bits = ~((u64)(EFER_SCE | EFER_LME | EFER_LMA));
#else
/* Enable syscall by default because its emulated by KVM */
static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
#endif
#define MAX_IO_MSRS 256

3
arch/x86/kvm/x86.c
View File

@ -6910,6 +6910,9 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_setup_xss_caps);
static void kvm_setup_efer_caps(void)
{
if (kvm_cpu_cap_has(X86_FEATURE_LM))
kvm_enable_efer_bits(EFER_LME | EFER_LMA);
if (kvm_cpu_cap_has(X86_FEATURE_NX))
kvm_enable_efer_bits(EFER_NX);