From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1F45524CEEA for ; Mon, 19 Jan 2026 16:00:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768838412; cv=none; b=hPV4wUhGBYzCDpzVHHMzN/ZXkHUerP7D1C0R9CL1ROf6Zpv3e8Ks8at1OsJWuDRJeBFESumQsvSCwBiCohRrot0OUEbscPNA2Fm+wC+ZQC25rb/BTpOs7gTObMquR8070EbeDVWaeb7y3ZoECZkB164tbHvssDAgxiKCchHohOc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768838412; c=relaxed/simple; bh=F/hzd277J2+sQOe030kb/TQGVNVvmxsy2dR5pXtGJjY=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=EAADg3WM2larhnLoEQa0Ab6/+rJIyhAO/jiD5cLVYEhMryIZ/1yEkMsyGWVP0O1IHx6q/669S3+xsPnRxIl2bag3fuGwpLRygi25N02vA/bJD24Rw+H9KA83SAGegF1HqtV83QEHJRgnSWZ4N6vn2PCR1MQiWDxUymcFsIw24DY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TsgPKOc4; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TsgPKOc4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3059EC116C6; Mon, 19 Jan 2026 16:00:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768838411; bh=F/hzd277J2+sQOe030kb/TQGVNVvmxsy2dR5pXtGJjY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=TsgPKOc44S6Z/alEp/gk9YGCDRxKyK6vtyhnkw2w1jofuf6z4XZlKsO6ZxmLDnLDn Edv8eY+V2+9oUBwV8whwJP2tIUBCwNyqYav+bJ/w9sdww6h1RLp7pKBRlDqozjSqUD Sk+BXso3Pjo+vhJrBkP3dn1e/aB7BaJmDWFpo04gOvIJYLzTKonfUnDglN9MehlQ/3 fON3yR0iiZv8bmBzRGxzdRfxMtsybEOG+3BvGP6Y5Jlr/qDoTPEOcspfllWHNc7GWz rZ3IkM5+XrI+gekcBF9Z5WzT17NdlzKHwaOtso5O4sa6W9n+5s+yZYX4s7SSw5K+xS fFe0O1Rrlxzlg== From: Sasha Levin To: stable@vger.kernel.org Cc: Sean Christopherson , Paolo Bonzini , Binbin Wu , Sasha Levin Subject: [PATCH 6.1.y] x86/fpu: Clear XSTATE_BV[i] in guest XSAVE state whenever XFD[i]=1 Date: Mon, 19 Jan 2026 11:00:09 -0500 Message-ID: <20260119160009.3069587-1-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <2026011919-violin-either-12c7@gregkh> References: <2026011919-violin-either-12c7@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Sean Christopherson [ Upstream commit b45f721775947a84996deb5c661602254ce25ce6 ] 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: 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 ---[ 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: 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 ---[ 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 Cc: stable@vger.kernel.org Fixes: 820a6ee944e7 ("kvm: x86: Add emulation for IA32_XFD", 2022-01-14) Signed-off-by: Sean Christopherson [Move clearing of XSTATE_BV from fpu_copy_uabi_to_guest_fpstate to kvm_vcpu_ioctl_x86_set_xsave. - Paolo] Reviewed-by: Binbin Wu Signed-off-by: Paolo Bonzini Signed-off-by: Sasha Levin --- 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 0068648cb7fbf..2805ce8f0259c 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -294,10 +294,29 @@ EXPORT_SYMBOL_GPL(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 its initial configuration. 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_GPL(fpu_update_guest_xfd); @@ -405,6 +424,13 @@ 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 1eb685d4b452e..3d63609b91def 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5341,9 +5341,18 @@ static void 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 0; + /* + * For backwards compatibility, do not expect disabled features to be in + * their initial state. XSTATE_BV[i] must still be cleared whenever + * XFD[i]=1, or 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, -- 2.51.0