x86/fpu: Sanitize xstateregs_set()

xstateregs_set() operates on a stopped task and tries to copy the provided
buffer into the task's fpu.state.xsave buffer.

Any error while copying or invalid state detected after copying results in
wiping the target task's FPU state completely including supervisor states.

That's just wrong. The caller supplied invalid data or has a problem with
unmapped memory, so there is absolutely no justification to corrupt the
target state.

Fix this with the following modifications:

 1) If data has to be copied from userspace, allocate a buffer and copy from
    user first.

 2) Use copy_kernel_to_xstate() unconditionally so that header checking
    works correctly.

 3) Return on error without corrupting the target state.

This prevents corrupting states and lets the caller deal with the problem
it caused in the first place.

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/20210623121452.214903673@linutronix.de

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

@@ -111,8 +111,4 @@ void copy_supervisor_to_kernel(struct xregs_state *xsave);
 void copy_dynamic_supervisor_to_kernel(struct xregs_state *xstate, u64 mask);
 void copy_kernel_to_dynamic_supervisor(struct xregs_state *xstate, u64 mask);
 
-
-/* Validate an xstate header supplied by userspace (ptrace or sigreturn) */
-int validate_user_xstate_header(const struct xstate_header *hdr);
-
 #endif

arch/x86/kernel/fpu/regset.c

@@ -2,11 +2,13 @@
 /*
  * FPU register's regset abstraction, for ptrace, core dumps, etc.
  */
+#include <linux/sched/task_stack.h>
+#include <linux/vmalloc.h>
+
 #include <asm/fpu/internal.h>
 #include <asm/fpu/signal.h>
 #include <asm/fpu/regset.h>
 #include <asm/fpu/xstate.h>
-#include <linux/sched/task_stack.h>
 
 /*
  * The xstateregs_active() routine is the same as the regset_fpregs_active() routine,
@@ -108,10 +110,10 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 		  const void *kbuf, const void __user *ubuf)
 {
 	struct fpu *fpu = &target->thread.fpu;
-	struct xregs_state *xsave;
+	struct xregs_state *tmpbuf = NULL;
 	int ret;
 
-	if (!boot_cpu_has(X86_FEATURE_XSAVE))
+	if (!cpu_feature_enabled(X86_FEATURE_XSAVE))
 		return -ENODEV;
 
 	/*
@@ -120,32 +122,22 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (pos != 0 || count != fpu_user_xstate_size)
 		return -EFAULT;
 
-	xsave = &fpu->state.xsave;
+	if (!kbuf) {
+		tmpbuf = vmalloc(count);
+		if (!tmpbuf)
+			return -ENOMEM;
 
-	fpu__prepare_write(fpu);
+		if (copy_from_user(tmpbuf, ubuf, count)) {
-
+			ret = -EFAULT;
-	if (using_compacted_format()) {
+			goto out;
-		if (kbuf)
+		}
-			ret = copy_kernel_to_xstate(xsave, kbuf);
-		else
-			ret = copy_user_to_xstate(xsave, ubuf);
-	} else {
-		ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, xsave, 0, -1);
-		if (!ret)
-			ret = validate_user_xstate_header(&xsave->header);
 	}
 
-	/*
+	fpu__prepare_write(fpu);
-	 * mxcsr reserved bits must be masked to zero for security reasons.
+	ret = copy_kernel_to_xstate(&fpu->state.xsave, kbuf ?: tmpbuf);
-	 */
-	xsave->i387.mxcsr &= mxcsr_feature_mask;
-
-	/*
-	 * In case of failure, mark all states as init:
-	 */
-	if (ret)
-		fpstate_init(&fpu->state);
 
+out:
+	vfree(tmpbuf);
 	return ret;
 }
 

arch/x86/kernel/fpu/xstate.c

@@ -543,7 +543,7 @@ int using_compacted_format(void)
 }
 
 /* Validate an xstate header supplied by userspace (ptrace or sigreturn) */
-int validate_user_xstate_header(const struct xstate_header *hdr)
+static int validate_user_xstate_header(const struct xstate_header *hdr)
 {
 	/* No unknown or supervisor features may be set */
 	if (hdr->xfeatures & ~xfeatures_mask_user())
@@ -1155,7 +1155,7 @@ void copy_xstate_to_kernel(struct membuf to, struct xregs_state *xsave)
 }
 
 /*
- * Convert from a ptrace standard-format kernel buffer to kernel XSAVES format
+ * Convert from a ptrace standard-format kernel buffer to kernel XSAVE[S] format
  * and copy to the target thread. This is called from xstateregs_set().
  */
 int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
@@ -1202,14 +1202,16 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
 	 */
 	xsave->header.xfeatures |= hdr.xfeatures;
 
+	/* mxcsr reserved bits must be masked to zero for historical reasons. */
+	xsave->i387.mxcsr &= mxcsr_feature_mask;
+
 	return 0;
 }
 
 /*
- * Convert from a ptrace or sigreturn standard-format user-space buffer to
+ * Convert from a sigreturn standard-format user-space buffer to kernel
- * kernel XSAVES format and copy to the target thread. This is called from
+ * XSAVE[S] format and copy to the target thread. This is called from the
- * xstateregs_set(), as well as potentially from the sigreturn() and
+ * sigreturn() and rt_sigreturn() system calls.
- * rt_sigreturn() system calls.
  */
 int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)
 {