[PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1

[Paolo Bonzini <pbonzini@redhat.com>]

From: Sean Christopherson <seanjc@google.com>

When loading guest XSAVE state via KVM_SET_XSAVE, and when updating XFD in
response to a guest WRMSR, clear XFD-disabled features in the saved (or to
be restored) XSTATE_BV to ensure KVM doesn't attempt to load state for
features that are disabled via the guest's XFD.  Because the kernel
executes XRSTOR with the guest's XFD, saving XSTATE_BV[i]=1 with XFD[i]=1
will cause XRSTOR to #NM and panic the kernel.

E.g. if fpu_update_guest_xfd() sets XFD without clearing XSTATE_BV:

  ------------[ cut here ]------------
  WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#29: amx_test/848
  Modules linked in: kvm_intel kvm irqbypass
  CPU: 29 UID: 1000 PID: 848 Comm: amx_test Not tainted 6.19.0-rc2-ffa07f7fd437-x86_amx_nm_xfd_non_init-vm #171 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:exc_device_not_available+0x101/0x110
  Call Trace:
   <TASK>
   asm_exc_device_not_available+0x1a/0x20
  RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90
   switch_fpu_return+0x4a/0xb0
   kvm_arch_vcpu_ioctl_run+0x1245/0x1e40 [kvm]
   kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]
   __x64_sys_ioctl+0x8f/0xd0
   do_syscall_64+0x62/0x940
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>
  ---[ end trace 0000000000000000 ]---

This can happen if the guest executes WRMSR(MSR_IA32_XFD) to set XFD[18] = 1,
and a host IRQ triggers kernel_fpu_begin() prior to the vmexit handler's
call to fpu_update_guest_xfd().

and if userspace stuffs XSTATE_BV[i]=1 via KVM_SET_XSAVE:

  ------------[ cut here ]------------
  WARNING: arch/x86/kernel/traps.c:1524 at exc_device_not_available+0x101/0x110, CPU#14: amx_test/867
  Modules linked in: kvm_intel kvm irqbypass
  CPU: 14 UID: 1000 PID: 867 Comm: amx_test Not tainted 6.19.0-rc2-2dace9faccd6-x86_amx_nm_xfd_non_init-vm #168 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:exc_device_not_available+0x101/0x110
  Call Trace:
   <TASK>
   asm_exc_device_not_available+0x1a/0x20
  RIP: 0010:restore_fpregs_from_fpstate+0x36/0x90
   fpu_swap_kvm_fpstate+0x6b/0x120
   kvm_load_guest_fpu+0x30/0x80 [kvm]
   kvm_arch_vcpu_ioctl_run+0x85/0x1e40 [kvm]
   kvm_vcpu_ioctl+0x2c3/0x8f0 [kvm]
   __x64_sys_ioctl+0x8f/0xd0
   do_syscall_64+0x62/0x940
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>
  ---[ end trace 0000000000000000 ]---

The new behavior is consistent with the AMX architecture.  Per Intel's SDM,
XSAVE saves XSTATE_BV as '0' for components that are disabled via XFD
(and non-compacted XSAVE saves the initial configuration of the state
component):

  If XSAVE, XSAVEC, XSAVEOPT, or XSAVES is saving the state component i,
  the instruction does not generate #NM when XCR0[i] = IA32_XFD[i] = 1;
  instead, it operates as if XINUSE[i] = 0 (and the state component was
  in its initial state): it saves bit i of XSTATE_BV field of the XSAVE
  header as 0; in addition, XSAVE saves the initial configuration of the
  state component (the other instructions do not save state component i).

Alternatively, KVM could always do XRSTOR with XFD=0, e.g. by using
a constant XFD based on the set of enabled features when XSAVEing for
a struct fpu_guest.  However, having XSTATE_BV[i]=1 for XFD-disabled
features can only happen in the above interrupt case, or in similar
scenarios involving preemption on preemptible kernels, because
fpu_swap_kvm_fpstate()'s call to save_fpregs_to_fpstate() saves the
outgoing FPU state with the current XFD; and that is (on all but the
first WRMSR to XFD) the guest XFD.

Therefore, XFD can only go out of sync with XSTATE_BV in the above
interrupt case, or in similar scenarios involving preemption on
preemptible kernels, and it we can consider it (de facto) part of KVM
ABI that KVM_GET_XSAVE returns XSTATE_BV[i]=0 for XFD-disabled features.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 820a6ee944e7 ("kvm: x86: Add emulation for IA32_XFD", 2022-01-14)
Signed-off-by: Sean Christopherson <seanjc@google.com>
[Move clearing of XSTATE_BV from fpu_copy_uabi_to_guest_fpstate
 to kvm_vcpu_ioctl_x86_set_xsave. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kernel/fpu/core.c | 32 +++++++++++++++++++++++++++++---
 arch/x86/kvm/x86.c         |  9 +++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index da233f20ae6f..166c380b0161 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -319,10 +319,29 @@ EXPORT_SYMBOL_FOR_KVM(fpu_enable_guest_xfd_features);
 #ifdef CONFIG_X86_64
 void fpu_update_guest_xfd(struct fpu_guest *guest_fpu, u64 xfd)
 {
+	struct fpstate *fpstate = guest_fpu->fpstate;
+
 	fpregs_lock();
-	guest_fpu->fpstate->xfd = xfd;
-	if (guest_fpu->fpstate->in_use)
-		xfd_update_state(guest_fpu->fpstate);
+
+	/*
+	 * KVM's guest ABI is that setting XFD[i]=1 *can* immediately revert
+	 * the save state to initialized.  Likewise, KVM_GET_XSAVE does the
+	 * same as XSAVE and returns XSTATE_BV[i]=0 whenever XFD[i]=1.
+	 *
+	 * If the guest's FPU state is in hardware, just update XFD: the XSAVE
+	 * in fpu_swap_kvm_fpstate will clear XSTATE_BV[i] whenever XFD[i]=1.
+	 *
+	 * If however the guest's FPU state is NOT resident in hardware, clear
+	 * disabled components in XSTATE_BV now, or a subsequent XRSTOR will
+	 * attempt to load disabled components and generate #NM _in the host_.
+	 */
+	if (xfd && test_thread_flag(TIF_NEED_FPU_LOAD))
+		fpstate->regs.xsave.header.xfeatures &= ~xfd;
+
+	fpstate->xfd = xfd;
+	if (fpstate->in_use)
+		xfd_update_state(fpstate);
+
 	fpregs_unlock();
 }
 EXPORT_SYMBOL_FOR_KVM(fpu_update_guest_xfd);
@@ -430,6 +449,13 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
 	if (ustate->xsave.header.xfeatures & ~xcr0)
 		return -EINVAL;
 
+	/*
+	 * Disabled features must be in their initial state, otherwise XRSTOR
+	 * causes an exception.
+	 */
+	if (WARN_ON_ONCE(ustate->xsave.header.xfeatures & kstate->xfd))
+		return -EINVAL;
+
 	/*
 	 * Nullify @vpkru to preserve its current value if PKRU's bit isn't set
 	 * in the header.  KVM's odd ABI is to leave PKRU untouched in this
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ff8812f3a129..c0416f53b5f5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5807,9 +5807,18 @@ static int kvm_vcpu_ioctl_x86_get_xsave(struct kvm_vcpu *vcpu,
 static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
 					struct kvm_xsave *guest_xsave)
 {
+	union fpregs_state *xstate = (union fpregs_state *)guest_xsave->region;
+
 	if (fpstate_is_confidential(&vcpu->arch.guest_fpu))
 		return vcpu->kvm->arch.has_protected_state ? -EINVAL : 0;
 
+	/*
+	 * Do not reject non-initialized disabled features for backwards
+	 * compatibility, but clear XSTATE_BV[i] whenever XFD[i]=1.
+	 * Otherwise, XRSTOR would cause a #NM.
+	 */
+	xstate->xsave.header.xfeatures &= ~vcpu->arch.guest_fpu.fpstate->xfd;
+
 	return fpu_copy_uabi_to_guest_fpstate(&vcpu->arch.guest_fpu,
 					      guest_xsave->region,
 					      kvm_caps.supported_xcr0,
-- 
2.52.0