x86/fpu: Rewrite xfpregs_set()

xfpregs_set() was incomprehensible.  Almost all of the complexity was due
to trying to support nonsensically sized writes or -EFAULT errors that
would have partially or completely overwritten the destination before
failing.  Nonsensically sized input would only have been possible using
PTRACE_SETREGSET on REGSET_XFP.  Fortunately, it appears (based on Debian
code search results) that no one uses that API at all, let alone with the
wrong sized buffer.  Failed user access can be handled more cleanly by
first copying to kernel memory.

Just rewrite it to require sensible input.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121452.504234607@linutronix.de

arch/x86/kernel/fpu/regset.c

@@ -47,30 +47,39 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
 		const void *kbuf, const void __user *ubuf)
 {
 	struct fpu *fpu = &target->thread.fpu;
+	struct user32_fxsr_struct newstate;
 	int ret;
 
-	if (!boot_cpu_has(X86_FEATURE_FXSR))
+	BUILD_BUG_ON(sizeof(newstate) != sizeof(struct fxregs_state));
+
+	if (!cpu_feature_enabled(X86_FEATURE_FXSR))
 		return -ENODEV;
 
+	/* No funny business with partial or oversized writes is permitted. */
+	if (pos != 0 || count != sizeof(newstate))
+		return -EINVAL;
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &newstate, 0, -1);
+	if (ret)
+		return ret;
+
+	/* Mask invalid MXCSR bits (for historical reasons). */
+	newstate.mxcsr &= mxcsr_feature_mask;
+
 	fpu__prepare_write(fpu);
-	fpstate_sanitize_xstate(fpu);
 
-	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
-				 &fpu->state.fxsave, 0, -1);
+	/* Copy the state  */
+	memcpy(&fpu->state.fxsave, &newstate, sizeof(newstate));
 
-	/*
-	 * mxcsr reserved bits must be masked to zero for security reasons.
-	 */
-	fpu->state.fxsave.mxcsr &= mxcsr_feature_mask;
+	/* Clear xmm8..15 */
+	BUILD_BUG_ON(sizeof(fpu->state.fxsave.xmm_space) != 16 * 16);
+	memset(&fpu->state.fxsave.xmm_space[8], 0, 8 * 16);
 
-	/*
-	 * update the header bits in the xsave header, indicating the
-	 * presence of FP and SSE state.
-	 */
-	if (boot_cpu_has(X86_FEATURE_XSAVE))
+	/* Mark FP and SSE as in use when XSAVE is enabled */
+	if (use_xsave())
 		fpu->state.xsave.header.xfeatures |= XFEATURE_MASK_FPSSE;
 
-	return ret;
+	return 0;
 }
 
 int xstateregs_get(struct task_struct *target, const struct user_regset *regset,