[patch 5/8] x86/fpu: Sanitize xstateregs_set()

[Thomas Gleixner]

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

Any error while copying or invalid state detected after copying results in
wiping the target tasks 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 wreckage the
target.

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 wreckaging the target state.

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

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/kernel/fpu/regset.c |   41 +++++++++++++++++------------------------
 arch/x86/kernel/fpu/xstate.c |   10 ++++++----
 2 files changed, 23 insertions(+), 28 deletions(-)

--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -6,8 +6,12 @@
 #include <asm/fpu/signal.h>
 #include <asm/fpu/regset.h>
 #include <asm/fpu/xstate.h>
+
+#include <linux/vmalloc.h>
+
 #include <linux/sched/task_stack.h>
 
+
 /*
  * The xstateregs_active() routine is the same as the regset_fpregs_active() routine,
  * as the "regset->n" for the xstate regset will be updated based on the feature
@@ -107,8 +111,8 @@ int xstateregs_set(struct task_struct *t
 		  unsigned int pos, unsigned int count,
 		  const void *kbuf, const void __user *ubuf)
 {
+	struct xregs_state *xsave, *xbuf = NULL;
 	struct fpu *fpu = &target->thread.fpu;
-	struct xregs_state *xsave;
 	int ret;
 
 	if (!boot_cpu_has(X86_FEATURE_XSAVE))
@@ -120,32 +124,21 @@ int xstateregs_set(struct task_struct *t
 	if (pos != 0 || count != fpu_user_xstate_size)
 		return -EFAULT;
 
-	xsave = &fpu->state.xsave;
-
-	fpu__prepare_write(fpu);
-
-	if (using_compacted_format()) {
-		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);
+	if (!kbuf) {
+		xbuf = vmalloc(count);
+		if (!xbuf)
+			return -ENOMEM;
+		ret = user_regset_copyin(&pos, &count, NULL, &ubuf, xbuf, 0, -1);
+		if (ret)
+			goto out;
 	}
 
-	/*
-	 * mxcsr reserved bits must be masked to zero for security reasons.
-	 */
-	xsave->i387.mxcsr &= mxcsr_feature_mask;
-
-	/*
-	 * In case of failure, mark all states as init:
-	 */
-	if (ret)
-		fpstate_init(&fpu->state);
+	xsave = &fpu->state.xsave;
+	fpu__prepare_write(fpu);
+	ret = copy_kernel_to_xstate(xsave, kbuf ? kbuf : xbuf);
 
+out:
+	vfree(xbuf);
 	return ret;
 }
 
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1172,14 +1172,16 @@ int copy_kernel_to_xstate(struct xregs_s
 	 */
 	xsave->header.xfeatures |= hdr.xfeatures;
 
+	/* mxcsr reserved bits must be masked to zero for security reasons. */
+	xsave->i387.mxcsr &= mxcsr_feature_mask;
+
 	return 0;
 }
 
 /*
- * Convert from a ptrace or sigreturn standard-format user-space buffer to
- * kernel XSAVES format and copy to the target thread. This is called from
- * xstateregs_set(), as well as potentially from the sigreturn() and
- * rt_sigreturn() system calls.
+ * Convert from a sigreturn standard-format user-space buffer to kernel
+ * XSAVES format and copy to the target thread. This is called from the
+ * sigreturn() and rt_sigreturn() system calls.
  */
 int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)
 {