[RFC PATCH 10/28] x86/xen: Avoid relocatable quantities in Xen ELF notes

Ard Biesheuvel
2024-09-25T15:02:16Z

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.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/kernel/vmlinux.lds.S | 13 +++++++++++++
arch/x86/platform/pvh/head.S | 6 +++---
arch/x86/tools/relocs.c | 1 +
arch/x86/xen/xen-head.S | 6 ++++--
4 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 00f82db7b3e1..52b8db931d0f 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -111,6 +111,19 @@ PHDRS {
SECTIONS
{
. = __START_KERNEL;
+
+#ifdef CONFIG_XEN_PV
+xen_elfnote_entry_offset =
+ ABSOLUTE(xen_elfnote_entry) + ABSOLUTE(startup_xen);
+xen_elfnote_hypercall_page_offset =
+ ABSOLUTE(xen_elfnote_hypercall_page) + ABSOLUTE(hypercall_page);
+#endif
+
+#ifdef CONFIG_PVH
+xen_elfnote_phys32_entry_offset =
+ ABSOLUTE(xen_elfnote_phys32_entry) + ABSOLUTE(pvh_start_xen - LOAD_OFFSET);
+#endif
+
#ifdef CONFIG_X86_32
phys_startup_32 = ABSOLUTE(startup_32 - LOAD_OFFSET);
#else
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 11245ecdc08d..adbf57e83e4e 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -50,7 +50,7 @@
#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

@@ -165,5 +165,5 @@ SYM_DATA_START_LOCAL(early_stack)
.fill BOOT_STACK_SIZE, 1, 0
SYM_DATA_END_LABEL(early_stack, SYM_L_LOCAL, early_stack_end)

- 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_offset - .)
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 942c029a5067..22c2d3f07a57 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -57,6 +57,7 @@ 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 faadac7c29e6..4d246a48a85f 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -88,7 +88,8 @@ 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_offset - .)
ELFNOTE(Xen, XEN_ELFNOTE_FEATURES, .ascii "!writable_page_tables")
ELFNOTE(Xen, XEN_ELFNOTE_PAE_MODE, .asciz "yes")
ELFNOTE(Xen, XEN_ELFNOTE_L1_MFN_VALID,
@@ -109,7 +110,8 @@ 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_offset - .)
ELFNOTE(Xen, XEN_ELFNOTE_SUPPORTED_FEATURES,
.long FEATURES_PV | FEATURES_PVH | FEATURES_DOM0)
ELFNOTE(Xen, XEN_ELFNOTE_LOADER, .asciz "generic")
--
2.46.0.792.g87dc391469-goog