From 698360199c9aa75f109c31814010d8bfb8abf019 Mon Sep 17 00:00:00 2001
From: Jonathon Hall <jonathon.hall@puri.sm>
Date: Thu, 10 Nov 2022 10:13:38 -0500
Subject: [PATCH] kexec-parse-boot: Ensure kernel/initrd paths exist in boot
 option

If a boot option doesn't refer to a valid file for the kernel/initrd,
ignore it.  Such an option is never bootable, because we would fail to
find the kernel/initrd.  This could happen if the path contained GRUB
variables, or specified a device that wasn't /boot, etc.

This is checked before expanding GRUB variables.  It's unlikely that
any kernel/initrd path containing variables would end up working when
all variables expand to nothing (since we do not handle GRUB
variables).

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>
---
 initrd/bin/kexec-parse-boot | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/initrd/bin/kexec-parse-boot b/initrd/bin/kexec-parse-boot
index b3dc6c0c..0aacd07e 100755
--- a/initrd/bin/kexec-parse-boot
+++ b/initrd/bin/kexec-parse-boot
@@ -28,15 +28,31 @@ fix_path() {
 	fi
 }
 
+# GRUB kernel lines (linux/multiboot) can include a command line.  Check whether
+# the file path exists in $bootdir.
+check_path() {
+	local checkpath firstval
+	checkpath="$1"
+	firstval="$(echo "$checkpath" | cut -d\  -f1)"
+	if ! [ -r "$bootdir$firstval" ]; then return 1; fi
+	return 0
+}
+
 echo_entry() {
 	if [ "$kexectype" = "elf" ]; then
 		if [ -z "$kernel" ]; then return; fi
 
 		fix_path $kernel
+		# The kernel must exist - if it doesn't, ignore this entry, it
+		# wouldn't work anyway.  This could happen if there was a
+		# GRUB variable in the kernel path, etc.
+		if ! check_path "$path"; then return; fi
 		entry="$name|$kexectype|kernel $path"
 		if [ -n "$initrd" ]; then
 			for init in $(echo $initrd | tr ',' ' '); do
 				fix_path $init
+				# The initrd must also exist
+				if ! check_path "$path"; then return; fi
 				entry="$entry|initrd $path"
 			done
 		fi
@@ -44,12 +60,18 @@ echo_entry() {
 			entry="$entry|append $append"
 		fi
 
+		# Double-expand here in case there are variables in the kernel
+		# parameters - some configs do this and can boot with empty
+		# expansions (Debian Live ISOs use this for loopback boots)
 		echo $(eval "echo \"$entry\"")
 	fi
 	if [ "$kexectype" = "multiboot" -o "$kexectype" = "xen" ]; then
 		if [ -z "$kernel" ]; then return; fi
 
 		fix_path $kernel
+		# The kernel must exist
+		if ! check_path "$path"; then return; fi
+		# Double-expand to clear variable expansions
 		echo $(eval "echo \"$name|$kexectype|kernel $path$modules\"")
 	fi
 }