x86/dumpstack: Fix x86_32 kernel_stack_pointer() previous stack access

On x86_32, when an interrupt happens from kernel space, SS and SP aren't
pushed and the existing stack is used.  So pt_regs is effectively two
words shorter, and the previous stack pointer is normally the memory
after the shortened pt_regs, aka '&regs->sp'.

But in the rare case where the interrupt hits right after the stack
pointer has been changed to point to an empty stack, like for example
when call_on_stack() is used, the address immediately after the
shortened pt_regs is no longer on the stack.  In that case, instead of
'&regs->sp', the previous stack pointer should be retrieved from the
beginning of the current stack page.

kernel_stack_pointer() wants to do that, but it forgets to dereference
the pointer.  So instead of returning a pointer to the previous stack,
it returns a pointer to the beginning of the current stack.

Note that it's probably outside of kernel_stack_pointer()'s scope to be
switching stacks at all.  The x86_64 version of this function doesn't do
it, and it would be better for the caller to do it if necessary.  But
that's a patch for another day.  This just fixes the original intent.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 0788aa6a23 ("x86: Prepare removal of previous_esp from i386 thread_info structure")
Link: http://lkml.kernel.org/r/472453d6e9f6a2d4ab16aaed4935f43117111566.1471535549.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/ptrace.c

@@ -173,8 +173,8 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
 		return sp;
 
 	prev_esp = (u32 *)(context);
-	if (prev_esp)
-		return (unsigned long)prev_esp;
+	if (*prev_esp)
+		return (unsigned long)*prev_esp;
 
 	return (unsigned long)regs;
 }