x86/fpu: Remove XSTATE_RESERVE

The original purpose of XSTATE_RESERVE was to carve out space
to store all of the possible extended state components that
get saved with the XSAVE instruction(s).

However, we are now almost entirely dynamically allocating
the buffers we use for XSAVE by placing them at the end of
the task_struct and them sizing them at boot.  The one
exception for that is the init_task.

The maximum extended state component size that we have today
is on systems with space for AVX-512 and Memory Protection
Keys: 2696 bytes.  We have reserved a PAGE_SIZE buffer in
the init_task via fpregs_state->__padding.

This check ensures that even if the component sizes or
layout were changed (which we do not expect), that we will
still not overflow the init_task's buffer.

In the case that we detect we might overflow the buffer,
we completely disable XSAVE support in the kernel and try
to boot as if we had 'legacy x87 FPU' support in place.
This is a crippled state without any of the XSAVE-enabled
features (MPX, AVX, etc...).  But, it at least let us
boot safely.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: dave@sr71.net
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20150902233125.D948D475@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 52 insertions, 8 deletions

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 2ada11c..769603a 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -312,24 +312,64 @@ static void __init setup_init_fpu_buf(void)
 /*
  * Calculate total size of enabled xstates in XCR0/xfeatures_mask.
  */
-static void __init init_xstate_size(void)
+static unsigned int __init calculate_xstate_size(void)
 {
 	unsigned int eax, ebx, ecx, edx;
+	unsigned int calculated_xstate_size;
 	int i;
 
 	if (!cpu_has_xsaves) {
 		cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
-		xstate_size = ebx;
-		return;
+		calculated_xstate_size = ebx;
+		return calculated_xstate_size;
 	}
 
-	xstate_size = FXSAVE_SIZE + XSAVE_HDR_SIZE;
+	calculated_xstate_size = FXSAVE_SIZE + XSAVE_HDR_SIZE;
 	for (i = 2; i < 64; i++) {
 		if (test_bit(i, (unsigned long *)&xfeatures_mask)) {
 			cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
-			xstate_size += eax;
+			calculated_xstate_size += eax;
 		}
 	}
+	return calculated_xstate_size;
+}
+
+/*
+ * Will the runtime-enumerated 'xstate_size' fit in the init
+ * task's statically-allocated buffer?
+ */
+static bool is_supported_xstate_size(unsigned int test_xstate_size)
+{
+	if (test_xstate_size <= sizeof(union fpregs_state))
+		return true;
+
+	pr_warn("x86/fpu: xstate buffer too small (%zu < %d), disabling xsave\n",
+			sizeof(union fpregs_state), test_xstate_size);
+	return false;
+}
+
+static int init_xstate_size(void)
+{
+	/* Recompute the context size for enabled features: */
+	unsigned int possible_xstate_size = calculate_xstate_size();
+
+	/* Ensure we have the space to store all enabled: */
+	if (!is_supported_xstate_size(possible_xstate_size))
+		return -EINVAL;
+
+	/*
+	 * The size is OK, we are definitely going to use xsave,
+	 * make it known to the world that we need more space.
+	 */
+	xstate_size = possible_xstate_size;
+	return 0;
+}
+
+void fpu__init_disable_system_xstate(void)
+{
+	xfeatures_mask = 0;
+	cr4_clear_bits(X86_CR4_OSXSAVE);
+	fpu__xstate_clear_all_cpu_caps();
 }
 
 /*
@@ -340,6 +380,7 @@ void __init fpu__init_system_xstate(void)
 {
 	unsigned int eax, ebx, ecx, edx;
 	static int on_boot_cpu = 1;
+	int err;
 
 	WARN_ON_FPU(!on_boot_cpu);
 	on_boot_cpu = 0;
@@ -367,9 +408,12 @@ void __init fpu__init_system_xstate(void)
 
 	/* Enable xstate instructions to be able to continue with initialization: */
 	fpu__init_cpu_xstate();
-
-	/* Recompute the context size for enabled features: */
-	init_xstate_size();
+	err = init_xstate_size();
+	if (err) {
+		/* something went wrong, boot without any XSAVE support */
+		fpu__init_disable_system_xstate();
+		return;
+	}
 
 	update_regset_xstate_info(xstate_size, xfeatures_mask);
 	fpu__init_prepare_fx_sw_frame();