x86/fpu: Extend fpu_xstate_prctl() with guest permissions

KVM requires a clear separation of host user space and guest permissions
for dynamic XSTATE components.

Add a guest permissions member to struct fpu and a separate set of prctl()
arguments: ARCH_GET_XCOMP_GUEST_PERM and ARCH_REQ_XCOMP_GUEST_PERM.

The semantics are equivalent to the host user space permission control
except for the following constraints:

  1) Permissions have to be requested before the first vCPU is created

  2) Permissions are frozen when the first vCPU is created to ensure
     consistency. Any attempt to expand permissions via the prctl() after
     that point is rejected.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jing Liu <jing2.liu@intel.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-2-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 8ea306b1bf8e..ab19b3d8b2f7 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -450,6 +450,8 @@ 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;
 }
 
 static inline void fpu_inherit_perms(struct fpu *dst_fpu)
@@ -460,6 +462,7 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
 		spin_lock_irq(&current->sighand->siglock);
 		/* Fork also inherits the permissions of the parent */
 		dst_fpu->perm = src_fpu->perm;
+		dst_fpu->guest_perm = src_fpu->guest_perm;
 		spin_unlock_irq(&current->sighand->siglock);
 	}
 }