x86/doublefault/32: Move #DF stack and TSS to cpu_entry_area

There are three problems with the current layout of the doublefault
stack and TSS.  First, the TSS is only cacheline-aligned, which is
not enough -- if the hardware portion of the TSS (struct x86_hw_tss)
crosses a page boundary, horrible things happen [0].  Second, the
stack and TSS are global, so simultaneous double faults on different
CPUs will cause massive corruption.  Third, the whole mechanism
won't work if user CR3 is loaded, resulting in a triple fault [1].

Let the doublefault stack and TSS share a page (which prevents the
TSS from spanning a page boundary), make it percpu, and move it into
cpu_entry_area.  Teach the stack dump code about the doublefault
stack.

[0] Real hardware will read past the end of the page onto the next
    *physical* page if a task switch happens.  Virtual machines may
    have any number of bugs, and I would consider it reasonable for
    a VM to summarily kill the guest if it tries to task-switch to
    a page-spanning TSS.

[1] Real hardware triple faults.  At least some VMs seem to hang.
    I'm not sure what's going on.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index ea866c7bf31d..804734058c77 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -65,6 +65,13 @@ enum exception_stack_ordering {
 
 #endif
 
+#ifdef CONFIG_X86_32
+struct doublefault_stack {
+	unsigned long stack[(PAGE_SIZE - sizeof(struct x86_hw_tss)) / sizeof(unsigned long)];
+	struct x86_hw_tss tss;
+} __aligned(PAGE_SIZE);
+#endif
+
 /*
  * cpu_entry_area is a percpu region that contains things needed by the CPU
  * and early entry/exit code.  Real types aren't used for all fields here
@@ -86,6 +93,11 @@ struct cpu_entry_area {
 #endif
 	struct entry_stack_page entry_stack_page;
 
+#ifdef CONFIG_X86_32
+	char guard_doublefault_stack[PAGE_SIZE];
+	struct doublefault_stack doublefault_stack;
+#endif
+
 	/*
 	 * On x86_64, the TSS is mapped RO.  On x86_32, it's mapped RW because
 	 * we need task switches to work, and task switches write to the TSS.