x86, relocs: Ignore relocations in .notes section

When building with CONFIG_XEN_PV=y, .text symbols are emitted into
the .notes section so that Xen can find the "startup_xen" entry point.
This information is used prior to booting the kernel, so relocations
are not useful. In fact, performing relocations against the .notes
section means that the KASLR base is exposed since /sys/kernel/notes
is world-readable.

To avoid leaking the KASLR base without breaking unprivileged tools that
are expecting to read /sys/kernel/notes, skip performing relocations in
the .notes section. The values readable in .notes are then identical to
those found in System.map.

Reported-by: Guixiong Wei <guixiongwei@gmail.com>
Closes: https://lore.kernel.org/all/20240218073501.54555-1-guixiongwei@gmail.com/
Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
Fixes: da1a679cde9b ("Add /sys/kernel/notes")
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index a3bae2b24626bc6b697b0da3117c804f973189cb..b029fb81ebeee0e8d5c703f5e64b7be0dbcc0c59 100644 (file)
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -653,6 +653,14 @@ static void print_absolute_relocs(void)
                if (!(sec_applies->shdr.sh_flags & SHF_ALLOC)) {
                        continue;
                }
+               /*
+                * Do not perform relocations in .notes section; any
+                * values there are meant for pre-boot consumption (e.g.
+                * startup_xen).
+                */
+               if (sec_applies->shdr.sh_type == SHT_NOTE) {
+                       continue;
+               }
                sh_symtab  = sec_symtab->symtab;
                sym_strtab = sec_symtab->link->strtab;
                for (j = 0; j < sec->shdr.sh_size/sizeof(Elf_Rel); j++) {