[PATCH v2] x86/kexec: Push kjump return address even for non-kjump kexec

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

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. Since commit 2cacf7f23a02 ("x86/kexec: Fix stack
and handling of re-entry point for ::preserve_context") 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.

Reported-by: Rohan Kakulawaram <rohanka@xxxxxxxxxx>
Tested-by: Rohan Kakulawaram <rohanka@xxxxxxxxxx>
Fixes: 2cacf7f23a02 ("x86/kexec: Fix stack and handling of re-entry point for ::preserve_context")
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
v2: Improve comment

 arch/x86/kernel/relocate_kernel_64.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 4ffba68dc57b..eaeb77464c06 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/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
 
-- 
2.43.0

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