x86/fpu: Make init_fpstate correct with optimized XSAVE

commit f9dfb5e390fab2df9f7944bb91e7705aba14cd26 upstream.

The XSAVE init code initializes all enabled and supported components with
XRSTOR(S) to init state. Then it XSAVEs the state of the components back
into init_fpstate which is used in several places to fill in the init state
of components.

This works correctly with XSAVE, but not with XSAVEOPT and XSAVES because
those use the init optimization and skip writing state of components which
are in init state. So init_fpstate.xsave still contains all zeroes after
this operation.

There are two ways to solve that:

   1) Use XSAVE unconditionally, but that requires to reshuffle the buffer when
      XSAVES is enabled because XSAVES uses compacted format.

   2) Save the components which are known to have a non-zero init state by other
      means.

Looking deeper, #2 is the right thing to do because all components the
kernel supports have all-zeroes init state except the legacy features (FP,
SSE). Those cannot be hard coded because the states are not identical on all
CPUs, but they can be saved with FXSAVE which avoids all conditionals.

Use FXSAVE to save the legacy FP/SSE components in init_fpstate along with
a BUILD_BUG_ON() which reminds developers to validate that a newly added
component has all zeroes init state. As a bonus remove the now unused
copy_xregs_to_kernel_booting() crutch.

The XSAVE and reshuffle method can still be implemented in the unlikely
case that components are added which have a non-zero init state and no
other means to save them. For now, FXSAVE is just simple and good enough.

  [ bp: Fix a typo or two in the text. ]

Fixes: 6bad06b76892 ("x86, xsave: Use xsaveopt in context-switch path when supported")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org
[ bp: 4.4 backport: Drop XFEATURE_MASK_{PKRU,PASID} which are not there yet. ]
Link: https://lkml.kernel.org/r/20210618143444.587311343@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 34 insertions, 3 deletions

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 3fa200ecca62..1ff1adbc843b 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -293,12 +293,31 @@ static void __init setup_xstate_comp(void)
 }
 
 /*
+ * All supported features have either init state all zeros or are
+ * handled in setup_init_fpu() individually. This is an explicit
+ * feature list and does not use XFEATURE_MASK*SUPPORTED to catch
+ * newly added supported features at build time and make people
+ * actually look at the init state for the new feature.
+ */
+#define XFEATURES_INIT_FPSTATE_HANDLED		\
+	(XFEATURE_MASK_FP |			\
+	 XFEATURE_MASK_SSE |			\
+	 XFEATURE_MASK_YMM |			\
+	 XFEATURE_MASK_OPMASK |			\
+	 XFEATURE_MASK_ZMM_Hi256 |		\
+	 XFEATURE_MASK_Hi16_ZMM	 |		\
+	 XFEATURE_MASK_BNDREGS |		\
+	 XFEATURE_MASK_BNDCSR)
+
+/*
  * setup the xstate image representing the init state
  */
 static void __init setup_init_fpu_buf(void)
 {
 	static int on_boot_cpu = 1;
 
+	BUILD_BUG_ON(XCNTXT_MASK != XFEATURES_INIT_FPSTATE_HANDLED);
+
 	WARN_ON_FPU(!on_boot_cpu);
 	on_boot_cpu = 0;
 
@@ -319,10 +338,22 @@ static void __init setup_init_fpu_buf(void)
 	copy_kernel_to_xregs_booting(&init_fpstate.xsave);
 
 	/*
-	 * Dump the init state again. This is to identify the init state
-	 * of any feature which is not represented by all zero's.
+	 * All components are now in init state. Read the state back so
+	 * that init_fpstate contains all non-zero init state. This only
+	 * works with XSAVE, but not with XSAVEOPT and XSAVES because
+	 * those use the init optimization which skips writing data for
+	 * components in init state.
+	 *
+	 * XSAVE could be used, but that would require to reshuffle the
+	 * data when XSAVES is available because XSAVES uses xstate
+	 * compaction. But doing so is a pointless exercise because most
+	 * components have an all zeros init state except for the legacy
+	 * ones (FP and SSE). Those can be saved with FXSAVE into the
+	 * legacy area. Adding new features requires to ensure that init
+	 * state is all zeroes or if not to add the necessary handling
+	 * here.
 	 */
-	copy_xregs_to_kernel_booting(&init_fpstate.xsave);
+	fxsave(&init_fpstate.fxsave);
 }
 
 static int xfeature_is_supervisor(int xfeature_nr)