From: Ard Biesheuvel <ardb@kernel.org>

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>
---
 arch/x86/kernel/vmlinux.lds.S | 19 +++++++++++++++++++
 arch/x86/platform/pvh/head.S  |  6 +++---
 arch/x86/tools/relocs.c       |  1 +
 arch/x86/xen/xen-head.S       |  6 ++++--
 4 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 6726be89b7a6..495f88c9d9f8 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -527,3 +527,22 @@ 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
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 7ca51a4da217..e6f39d77f0b4 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/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 - .)
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index c101bed61940..3ede19ca8432 100644
--- a/arch/x86/tools/relocs.c
+++ b/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_)",
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 758bcd47b72d..7f6c69dbb816 100644
--- a/arch/x86/xen/xen-head.S
+++ b/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")
-- 
2.47.0.rc0.187.ge670bccf7e-goog

On 2024-10-09 12:04, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> 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>

Thanks,
Jason