From faf0ed487415f76fe4acf7980ce360901f5e1698 Mon Sep 17 00:00:00 2001 From: Ard Biesheuvel Date: Thu, 5 Dec 2024 12:28:12 +0100 Subject: [PATCH] x86/boot: Reject absolute references in .head.text The .head.text section used to contain asm code that bootstrapped the page tables and switched to the kernel virtual address space before executing C code. The asm code carefully avoided dereferencing absolute symbol references, as those will fault before the page tables are installed. Today, the .head.text section contains lots of C code too, and getting the compiler to reason about absolute addresses taken from, e.g., section markers such as _text[] or _end[] but never use such absolute references to access global variables [*] is intractible. So instead, forbid the use of absolute references in .head.text entirely, and rely on explicit arithmetic involving VA-to-PA offsets generated by the asm startup code to construct virtual addresses where needed (e.g., to construct the page tables). Note that the 'relocs' tool is only used on the core kernel image when building a relocatable image, but this is the default, and so adding the check there is sufficient to catch new occurrences of code that use absolute references before the kernel mapping is up. [*] it is feasible when using PIC codegen but there is strong pushback to using this for all of the core kernel, and using it only for .head.text is not straight-forward. Signed-off-by: Ard Biesheuvel Signed-off-by: Ingo Molnar Cc: Linus Torvalds Cc: "H. Peter Anvin" Link: https://lore.kernel.org/r/20241205112804.3416920-16-ardb+git@google.com --- arch/x86/tools/relocs.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c index 27441e5863b23b..e937be979ec86d 100644 --- a/arch/x86/tools/relocs.c +++ b/arch/x86/tools/relocs.c @@ -841,10 +841,10 @@ static int is_percpu_sym(ElfW(Sym) *sym, const char *symname) static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym, const char *symname) { + int headtext = !strcmp(sec_name(sec->shdr.sh_info), ".head.text"); unsigned r_type = ELF64_R_TYPE(rel->r_info); ElfW(Addr) offset = rel->r_offset; int shn_abs = (sym->st_shndx == SHN_ABS) && !is_reloc(S_REL, symname); - if (sym->st_shndx == SHN_UNDEF) return 0; @@ -900,6 +900,12 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym, break; } + if (headtext) { + die("Absolute reference to symbol '%s' not permitted in .head.text\n", + symname); + break; + } + /* * Relocation offsets for 64 bit kernels are output * as 32 bits and sign extended back to 64 bits when