x86/kexec: Fix stack and handling of re-entry point for ::preserve_co…

…ntext

A ::preserve_context kimage can be invoked more than once, and the entry point
can be different every time. When the callee returns to the kernel, it leaves
the address of its entry point for next time on the stack.

That being the case, one might reasonably assume that the caller would
allocate space for it on the stack frame before actually performing the 'call'
into the callee.

Apparently not, though. Ever since the kjump code was first added in 2009, it
has set up a *new* stack at the top of the swap_page scratch page, then just
performed the 'call' without allocating any space for the re-entry address to
be returned. It then reads the re-entry point for next time from 0(%rsp) which
is actually the first qword of the page *after* the swap page, which might not
exist at all! And if the callee has written to that, then it will have
corrupted memory it doesn't own.

Correct this by pushing the entry point of the callee onto the stack before
calling it. The callee may then adjust it, or not, as it sees fit, and
subsequent invocations should work correctly either way.

Remove a stray push of zero to the *relocate_kernel* stack, which may have
been intended for this purpose, but which was actually just noise.

Also, loading the stack for the callee relied on the address of the swap page
being in %r10 without ever documenting that fact. Recent code changes made
that no longer true, so load it directly from the local kexec_pa_swap_page
variable instead.

Fixes: b3adaba ("x86/kexec: Drop page_list argument from relocate_kernel()")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250109140757.2841269-5-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -113,8 +113,6 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	 * %r13 original CR4 when relocate_kernel() was invoked
 	 */
 
-	/* set return address to 0 if not preserving context */
-	pushq	$0
 	/* store the start address on the stack */
 	pushq   %rdx
 
@@ -208,12 +206,19 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 
 .Lrelocate:
 	popq	%rdx
+
+	/* Use the swap page for the callee's stack */
+	movq	kexec_pa_swap_page(%rip), %r10
 	leaq	PAGE_SIZE(%r10), %rsp
+
+	/* push the existing entry point onto the callee's stack */
+	pushq	%rdx
+
 	ANNOTATE_RETPOLINE_SAFE
 	call	*%rdx
 
 	/* get the re-entry point of the peer system */
-	movq	0(%rsp), %rbp
+	popq	%rbp
 	leaq	relocate_kernel(%rip), %r8
 	movq	kexec_pa_swap_page(%rip), %r10
 	movq	pa_backup_pages_map(%rip), %rdi
@@ -247,6 +252,7 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
 	lgdt    saved_context_gdt_desc(%rax)
 #endif
 
+	/* relocate_kernel() returns the re-entry point for next time */
 	movq	%rbp, %rax
 
 	popf