x86/kexec: Push kjump return address even for non-kjump kexec

The version of purgatory code shipped by kexec-tools attempts to look above
the top of its stack to find a return address for a kjump, even in a non-kjump
kexec.

After the commit in Fixes: the word above the stack might not be there,
leading to a fault (which is at least now caught by my exception-handling code
in kexec).

That commit fixed things for the actual kjump path, but no longer
"gratuitously" pushes the unused return address to the stack in the non-kjump
path. Put that *back* in the non-kjump path, to prevent purgatory from
crashing when trying to access it.

Fixes: 2cacf7f ("x86/kexec: Fix stack and handling of re-entry point for ::preserve_context")
Reported-by: Rohan Kakulawaram <rohanka@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Rohan Kakulawaram <rohanka@google.com>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/32d627134143ffd957891cb697138e839c623211.camel@infradead.org

Author: dwmw2

arch/x86/kernel/relocate_kernel_64.S

@@ -136,6 +136,14 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	 * %r13 original CR4 when relocate_kernel() was invoked
 	 */
 
+	/*
+	 * Set return address to 0 if not preserving context. The purgatory
+	 * shipped in kexec-tools will unconditionally look for the return
+	 * address on the stack and set a kexec_jump_back_entry= command
+	 * line option if it's non-zero. There's no other way that it can
+	 * tell a preserve-context (kjump) kexec from a normal one.
+	 */
+	pushq	$0
 	/* store the start address on the stack */
 	pushq   %rdx