x86/fpu: Add members to struct fpu to cache permission information

Dynamically enabled features can be requested by any thread of a running
process at any time. The request does neither enable the feature nor
allocate larger buffers. It just stores the permission to use the feature
by adding the features to the permission bitmap and by calculating the
required sizes for kernel and user space.

The reallocation of the kernel buffer happens when the feature is used
for the first time which is caught by an exception. The permission
bitmap is then checked and if the feature is permitted, then it becomes
fully enabled. If not, the task dies similarly to a task which uses an
undefined instruction.

The size information is precomputed to allow proper sigaltstack size checks
once the feature is permitted, but not yet in use because otherwise this
would open race windows where too small stacks could be installed causing
a later fail on signal delivery.

Initialize them to the default feature set and sizes.

Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Chang S. Bae <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/fpu/types.h

···
 	/* @regs is dynamically sized! Don't add anything after @regs! */
 } __aligned(64);
 
+struct fpu_state_perm {
+	/*
+	 * @__state_perm:
+	 *
+	 * This bitmap indicates the permission for state components, which
+	 * are available to a thread group. The permission prctl() sets the
+	 * enabled state bits in thread_group_leader()->thread.fpu.
+	 *
+	 * All run time operations use the per thread information in the
+	 * currently active fpu.fpstate which contains the xfeature masks
+	 * and sizes for kernel and user space.
+	 *
+	 * This master permission field is only to be used when
+	 * task.fpu.fpstate based checks fail to validate whether the task
+	 * is allowed to expand it's xfeatures set which requires to
+	 * allocate a larger sized fpstate buffer.
+	 *
+	 * Do not access this field directly.  Use the provided helper
+	 * function. Unlocked access is possible for quick checks.
+	 */
+	u64				__state_perm;
+
+	/*
+	 * @__state_size:
+	 *
+	 * The size required for @__state_perm. Only valid to access
+	 * with sighand locked.
+	 */
+	unsigned int			__state_size;
+
+	/*
+	 * @__user_state_size:
+	 *
+	 * The size required for @__state_perm user part. Only valid to
+	 * access with sighand locked.
+	 */
+	unsigned int			__user_state_size;
+};
+
 /*
  * Highest level per task FPU state data structure that
  * contains the FPU register state plus various FPU
···
 	 * used only for KVM support to swap out the regular task fpstate.
 	 */
 	struct fpstate			*__task_fpstate;
+
+	/*
+	 * @perm:
+	 *
+	 * Permission related information
+	 */
+	struct fpu_state_perm		perm;
 
 	/*
 	 * @__fpstate:

arch/x86/kernel/fpu/core.c

···
 	/* Set the fpstate pointer to the default fpstate */
 	fpu->fpstate = &fpu->__fpstate;
 	__fpstate_reset(fpu->fpstate);
+
+	/* Initialize the permission related info in 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;
 }
 
 /* Clone current's FPU state on fork */