x86/kexec: Fix bug with call depth tracking

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From: David Kaplan <david.kaplan@xxxxxxx>

commit 93c1800b3799f17375989b0daf76497dd3e80922 upstream.

The call to cc_platform_has() triggers a fault and system crash if call depth
tracking is active because the GS segment has been reset by load_segments() and
GS_BASE is now 0 but call depth tracking uses per-CPU variables to operate.

Call cc_platform_has() earlier in the function when GS is still valid.

  [ bp: Massage. ]

Fixes: 5d8213864ade ("x86/retbleed: Add SKL return thunk")
Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: <stable@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240603083036.637-1-bp@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kernel/machine_kexec_64.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -295,8 +295,15 @@ void machine_kexec_cleanup(struct kimage
 void machine_kexec(struct kimage *image)
 {
 	unsigned long page_list[PAGES_NR];
-	void *control_page;
+	unsigned int host_mem_enc_active;
 	int save_ftrace_enabled;
+	void *control_page;
+
+	/*
+	 * This must be done before load_segments() since if call depth tracking
+	 * is used then GS must be valid to make any function calls.
+	 */
+	host_mem_enc_active = cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT);
 
 #ifdef CONFIG_KEXEC_JUMP
 	if (image->preserve_context)
@@ -358,7 +365,7 @@ void machine_kexec(struct kimage *image)
 				       (unsigned long)page_list,
 				       image->start,
 				       image->preserve_context,
-				       cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT));
+				       host_mem_enc_active);
 
 #ifdef CONFIG_KEXEC_JUMP
 	if (image->preserve_context)


Patches currently in stable-queue which might be from david.kaplan@xxxxxxx are

queue-6.9/x86-kexec-fix-bug-with-call-depth-tracking.patch




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