x86/xen: Avoid relocatable quantities in Xen ELF notes

Xen puts virtual and physical addresses into ELF notes that are treated
by the linker as relocatable by default. Doing so is not only pointless,
given that the ELF notes are only intended for consumption by Xen before
the kernel boots. It is also a KASLR leak, given that the kernel's ELF
notes are exposed via the world readable /sys/kernel/notes.

So emit these constants in a way that prevents the linker from marking
them as relocatable. This involves place-relative relocations (which
subtract their own virtual address from the symbol value) and linker
provided absolute symbols that add the address of the place to the
desired value.

Tested-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Message-ID: <20241009160438.3884381-11-ardb+git@google.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

Author: ardbiesheuvel

arch/x86/kernel/vmlinux.lds.S

@@ -528,3 +528,22 @@ INIT_PER_CPU(irq_stack_backing_store);
 #endif
 
 #endif /* CONFIG_X86_64 */
+
+/*
+ * The symbols below are referenced using relative relocations in the
+ * respective ELF notes. This produces build time constants that the
+ * linker will never mark as relocatable. (Using just ABSOLUTE() is not
+ * sufficient for that).
+ */
+#ifdef CONFIG_XEN
+#ifdef CONFIG_XEN_PV
+xen_elfnote_entry_value =
+	ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
+#endif
+xen_elfnote_hypercall_page_value =
+	ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
+#endif
+#ifdef CONFIG_PVH
+xen_elfnote_phys32_entry_value =
+	ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);
+#endif

arch/x86/platform/pvh/head.S

@@ -52,7 +52,7 @@
 #define PVH_CS_SEL		(PVH_GDT_ENTRY_CS * 8)
 #define PVH_DS_SEL		(PVH_GDT_ENTRY_DS * 8)
 
-SYM_CODE_START_LOCAL(pvh_start_xen)
+SYM_CODE_START(pvh_start_xen)
 	UNWIND_HINT_END_OF_STACK
 	cld
 
@@ -300,5 +300,5 @@ SYM_DATA_END(pvh_level2_kernel_pgt)
 		     .long KERNEL_IMAGE_SIZE - 1)
 #endif
 
-	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY,
-	             _ASM_PTR (pvh_start_xen - __START_KERNEL_map))
+	ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .global xen_elfnote_phys32_entry;
+		xen_elfnote_phys32_entry: _ASM_PTR xen_elfnote_phys32_entry_value - .)

arch/x86/tools/relocs.c

@@ -56,6 +56,7 @@ static const char * const	sym_regex_kernel[S_NSYMTYPES] = {
 	[S_ABS] =
 	"^(xen_irq_disable_direct_reloc$|"
 	"xen_save_fl_direct_reloc$|"
+	"xen_elfnote_.+_offset$|"
 	"VDSO|"
 	"__kcfi_typeid_|"
 	"__crc_)",

arch/x86/xen/xen-head.S

@@ -94,7 +94,8 @@ SYM_CODE_END(xen_cpu_bringup_again)
 	ELFNOTE(Xen, XEN_ELFNOTE_VIRT_BASE,      _ASM_PTR __START_KERNEL_map)
 	/* Map the p2m table to a 512GB-aligned user address. */
 	ELFNOTE(Xen, XEN_ELFNOTE_INIT_P2M,       .quad (PUD_SIZE * PTRS_PER_PUD))
-	ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          _ASM_PTR startup_xen)
+	ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          .globl xen_elfnote_entry;
+		xen_elfnote_entry: _ASM_PTR xen_elfnote_entry_value - .)
 	ELFNOTE(Xen, XEN_ELFNOTE_FEATURES,       .ascii "!writable_page_tables")
 	ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE,       .asciz "yes")
 	ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID,
@@ -115,7 +116,8 @@ SYM_CODE_END(xen_cpu_bringup_again)
 #else
 # define FEATURES_DOM0 0
 #endif
-	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, _ASM_PTR hypercall_page)
+	ELFNOTE(Xen, XEN_ELFNOTE_HYPERCALL_PAGE, .globl xen_elfnote_hypercall_page;
+		xen_elfnote_hypercall_page: _ASM_PTR xen_elfnote_hypercall_page_value - .)
 	ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES,
 		.long FEATURES_PV | FEATURES_PVH | FEATURES_DOM0)
 	ELFNOTE(Xen, XEN_ELFNOTE_LOADER,         .asciz "generic")