[PATCH 33/37] x86, fpu: allow setting of XSAVE state

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

We want to modify the Protection Key rights inside the kernel, so
we need to change PKRU's contents.  But, if we do a plain
'wrpkru', when we return to userspace we might do an XRSTOR and
wipe out the kernel's 'wrpkru'.  So, we need to go after PKRU in
the xsave buffer.

We do this by looking up the location of a given state in the
XSAVE buffer, filling in the state, and then ensuring that the
hardware knows that state is present there (basically that the
'init optimization' is not in place).


Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---

 b/arch/x86/kernel/fpu/xstate.c |  109 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 107 insertions(+), 2 deletions(-)

diff -puN arch/x86/kernel/fpu/xstate.c~pkey-xsave-set arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~pkey-xsave-set	2015-11-16 12:35:50.714869761 -0800
+++ b/arch/x86/kernel/fpu/xstate.c	2015-11-16 12:35:50.717869897 -0800
@@ -679,6 +679,19 @@ void fpu__resume_cpu(void)
 }
 
 /*
+ * Given an xstate feature mask, calculate where in the xsave
+ * buffer the state is.  Callers should ensure that the buffer
+ * is valid.
+ *
+ * Note: does not work for compacted buffers.
+ */
+void *__raw_xsave_addr(struct xregs_state *xsave, int xstate_feature_mask)
+{
+	int feature_nr = fls64(xstate_feature_mask) - 1;
+
+	return (void *)xsave + xstate_comp_offsets[feature_nr];
+}
+/*
  * Given the xsave area and a state inside, this function returns the
  * address of the state.
  *
@@ -698,7 +711,6 @@ void fpu__resume_cpu(void)
  */
 void *get_xsave_addr(struct xregs_state *xsave, int xstate_feature)
 {
-	int feature_nr = fls64(xstate_feature) - 1;
 	/*
 	 * Do we even *have* xsave state?
 	 */
@@ -727,7 +739,7 @@ void *get_xsave_addr(struct xregs_state
 	if (!(xsave->header.xfeatures & xstate_feature))
 		return NULL;
 
-	return (void *)xsave + xstate_comp_offsets[feature_nr];
+	return __raw_xsave_addr(xsave, xstate_feature);
 }
 EXPORT_SYMBOL_GPL(get_xsave_addr);
 
@@ -762,3 +774,96 @@ const void *get_xsave_field_ptr(int xsav
 
 	return get_xsave_addr(&fpu->state.xsave, xsave_state);
 }
+
+
+/*
+ * Set xfeatures (aka XSTATE_BV) bit for a feature that we want
+ * to take out of its "init state".  This will ensure that an
+ * XRSTOR actually restores the state.
+ */
+static void fpu__xfeature_set_non_init(struct xregs_state *xsave,
+		int xstate_feature_mask)
+{
+	xsave->header.xfeatures |= xstate_feature_mask;
+}
+
+/*
+ * This function is safe to call whether the FPU is in use or not.
+ *
+ * Note that this only works on the current task.
+ *
+ * Inputs:
+ *	@xsave_state: state which is defined in xsave.h (e.g. XFEATURE_MASK_FP,
+ *	XFEATURE_MASK_SSE, etc...)
+ *	@xsave_state_ptr: a pointer to a copy of the state that you would
+ *	like written in to the current task's FPU xsave state.  This pointer
+ *	must not be located in the current tasks's xsave area.
+ * Output:
+ *	address of the state in the xsave area or NULL if the state
+ *	is not present or is in its 'init state'.
+ */
+static void fpu__xfeature_set_state(int xstate_feature_mask,
+		void *xstate_feature_src, size_t len)
+{
+	struct xregs_state *xsave = &current->thread.fpu.state.xsave;
+	struct fpu *fpu = &current->thread.fpu;
+	void *dst;
+
+	if (!boot_cpu_has(X86_FEATURE_XSAVE)) {
+		WARN_ONCE(1, "%s() attempted with no xsave support", __func__);
+		return;
+	}
+	/*
+	 * Move the CPU FPU state in to the buffer at 'fpu'.
+	 * Activate it if not active.  Now we can get the old
+	 * pkru state out of the buffer.
+	 */
+	fpu__activate_fpstate_read(fpu);
+	/*
+	 * Move the CPU FPU state in to the buffer at 'fpu'.  Now
+	 * we can modify the buffer.
+	 */
+	fpu__save(fpu);
+	/*
+	 * This method *WILL* *NOT* work for compact-format
+	 * buffers.  If the 'xstate_feature_mask' is unset in
+	 * xcomp_bv then we may need to move other feature state
+	 * "up" in the buffer.
+	 */
+	if (xsave->header.xcomp_bv & xstate_feature_mask) {
+		WARN_ON_ONCE(1);
+		return;
+	}
+
+	/* find the location in the xsave buffer of the desired state */
+	dst = __raw_xsave_addr(&fpu->state.xsave, xstate_feature_mask);
+
+	/*
+	 * Make sure that the pointer being passed in did not
+	 * come from the xsave buffer itself.
+	 */
+	WARN_ONCE(xstate_feature_src == dst, "set from xsave buffer itself");
+
+	/* tell the FPU code that we are writing to the FPU state */
+	fpu__activate_fpstate_write(fpu);
+
+	/* put the caller-requested data in the location */
+	memcpy(dst, xstate_feature_src, len);
+
+	/*
+	 * Mark the xfeature so that the CPU knows there is state
+	 * in the buffer now.
+	 */
+	fpu__xfeature_set_non_init(xsave, xstate_feature_mask);
+
+	/*
+	 * The fpu state now has an updated copy of the state,
+	 * but the registers may still be out of date.  Update
+	 * them with an XRSTOR.
+	 */
+	preempt_disable();
+	fpu__restore(fpu);
+	preempt_enable();
+
+	return 0;
+}
_
--
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