[PATCH] x86/fpu/xstate: Fix inconsistencies in guest FPU xfeatures

[Chao Gao]

Guest FPUs manage vCPU FPU states. They are allocated via
fpu_alloc_guest_fpstate() and are resized in fpstate_realloc() when XFD
features are enabled.

Since the introduction of guest FPUs, there have been inconsistencies in
the kernel buffer size and xfeatures:

1.fpu_alloc_guest_fpstate() uses fpu_user_cfg since its introduction. See:

  commit 69f6ed1d14c6 ("x86/fpu: Provide infrastructure for KVM FPU cleanup")
  commit 36487e6228c4 ("x86/fpu: Prepare guest FPU for dynamically enabled FPU features")

2.__fpstate_reset() references fpu_kernel_cfg to set storage attributes.

3.fpu->guest_perm uses fpu_kernel_cfg, affecting fpstate_realloc().

A recent commit in the tip-tree [1] partially addressed the inconsistency
between (1) and (3) by using fpu_kernel_cfg for size calculation in (1),
but left fpu_guest->xfeatures and fpu_guest->perm still referencing
fpu_user_cfg.

The inconsistencies within fpu_alloc_guest_fpstate() and across the
mentioned functions cause confusion.

Fix them by using fpu_kernel_cfg consistently in fpu_alloc_guest_fpstate(),
except for fields related to the UABI buffer. Referencing fpu_kernel_cfg
won't impact functionalities, as
1. fpu_guest->perm is overwritten shortly in fpu_init_guest_permissions()
   with fpstate->guest_perm, which already uses fpu_kernel_cfg.
2. fpu_guest->xfeatures is solely used to check if XFD features are enabled.
   Including supervisor xfeatures doesn't affect the check.

Link: https://lore.kernel.org/all/20250218141045.85201-1-stanspas@xxxxxxxxx/ [1]
Fixes: 36487e6228c4 ("x86/fpu: Prepare guest FPU for dynamically enabled FPU features")
Suggested-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
 arch/x86/kernel/fpu/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 422c98ca6eb8..1b734a9ff088 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -239,8 +239,8 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
 	fpstate->is_guest	= true;
 
 	gfpu->fpstate		= fpstate;
-	gfpu->xfeatures		= fpu_user_cfg.default_features;
-	gfpu->perm		= fpu_user_cfg.default_features;
+	gfpu->xfeatures		= fpu_kernel_cfg.default_features;
+	gfpu->perm		= fpu_kernel_cfg.default_features;
 
 	/*
 	 * KVM sets the FP+SSE bits in the XSAVE header when copying FPU state

base-commit: d08c0d55110b7cbac186e5fa94b0c6d5f4d7905e
-- 
2.47.1