linux-kernel - [PATCH v8 2/6] x86/fpu: Initialize guest FPU permissions from guest defaults

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Message-ID: <20250522151031.426788-3-chao.gao@intel.com>
Date: Thu, 22 May 2025 08:10:05 -0700
From: Chao Gao <chao.gao@...el.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	tglx@...utronix.de,
	dave.hansen@...el.com,
	seanjc@...gle.com,
	pbonzini@...hat.com
Cc: peterz@...radead.org,
	rick.p.edgecombe@...el.com,
	weijiang.yang@...el.com,
	john.allen@....com,
	bp@...en8.de,
	chang.seok.bae@...el.com,
	xin3.li@...el.com,
	Chao Gao <chao.gao@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Stanislav Spassov <stanspas@...zon.de>,
	Eric Biggers <ebiggers@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Kees Cook <kees@...nel.org>
Subject: [PATCH v8 2/6] x86/fpu: Initialize guest FPU permissions from guest defaults

Currently, fpu->guest_perm is copied from fpu->perm, which is derived from
fpu_kernel_cfg.default_features.

Guest defaults were introduced to differentiate the features and sizes of
host and guest FPUs. Copying guest FPU permissions from the host will lead
to inconsistencies between the guest default features and permissions.

Initialize guest FPU permissions from guest defaults instead of host
defaults. This ensures that any changes to guest default features are
automatically reflected in guest permissions, which in turn guarantees
that fpstate_realloc() allocates a correctly sized XSAVE buffer for guest
FPUs.

Suggested-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
v8: Refine the comment above fpu->guest_perm.__user_state_size
v6: Drop vcpu_fpu_config.user_* and collect reviews (Rick)
---
 arch/x86/kernel/fpu/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 2cd5e1910ff8..c69432d0ee41 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -553,8 +553,14 @@ void fpstate_reset(struct fpu *fpu)
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
 	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
 	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
-	/* Same defaults for guests */
-	fpu->guest_perm = fpu->perm;
+
+	fpu->guest_perm.__state_perm	= guest_default_cfg.features;
+	fpu->guest_perm.__state_size	= guest_default_cfg.size;
+	/*
+	 * User features and sizes are always identical between host and
+	 * guest FPUs, which allows for common guest and userspace ABI.
+	 */
+	fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
 }
 
 static inline void fpu_inherit_perms(struct fpu *dst_fpu)
-- 
2.47.1