From: Ard Biesheuvel <ardb@xxxxxxxxxx> This is a follow-up to my RFC [0] that proposed to build the entire core kernel with -fPIC, to reduce the likelihood that code that runs extremely early from the 1:1 mapping of memory will misbehave. This is needed to address reports that SEV boot on Clang built kernels is broken, due to the fact that this early code attempts to access virtual kernel address that are not mapped yet. Kevin has suggested some workarounds to this [1] but this is really something that requires a more rigorous approach, rather than addressing a couple of symptoms of the underlying defect. As it turns out, the use of fPIE for the entire kernel is neither necessary nor sufficient, and has its own set of problems, including the fact that the PIE small C code model uses FS rather than GS for the per-CPU register, and only recent GCC and Clang versions permit this to be overridden on the command line. But the real problem is that even position independent code is not guaranteed to execute correctly at any offset unless all statically initialized pointer variables use the same translation as the code. So instead, this v2 and later proposes another solution, taking the following approach: - clean up and refactor the startup code so that the primary startup code executes from the 1:1 mapping but nothing else; - define a new text section type .pi.text and enforce that it can only call into other .pi.text sections; - (tbd) require that objects containing .pi.text sections are built with -fPIC, and disallow any absolute references from such objects. The latter point is not implemented yet in this v3, but this could be done rather straight-forwardly. (The EFI stub already does something similar across all architectures) Changes since v2: [2] - move command line parsing out of early startup code entirely - fix LTO and instrumentation related build warnings reported by Nathan - omit PTI related PGD/P4D setters when creating the early page tables, instead of pulling that code into the 'early' set [0] https://lkml.kernel.org/r/20240122090851.851120-7-ardb%2Bgit%40google.com [1] https://lore.kernel.org/all/20240111223650.3502633-1-kevinloughlin@xxxxxxxxxx/T/#u [2] https://lkml.kernel.org/r/20240125112818.2016733-19-ardb%2Bgit%40google.com Cc: Kevin Loughlin <kevinloughlin@xxxxxxxxxx> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx> Cc: Dionna Glaze <dionnaglaze@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Nathan Chancellor <nathan@xxxxxxxxxx> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Cc: Justin Stitt <justinstitt@xxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-arch@xxxxxxxxxxxxxxx Cc: llvm@xxxxxxxxxxxxxxx Ard Biesheuvel (19): efi/libstub: Add generic support for parsing mem_encrypt= x86/boot: Move mem_encrypt= parsing to the decompressor x86/startup_64: Drop long return to initial_code pointer x86/startup_64: Simplify calculation of initial page table address x86/startup_64: Simplify CR4 handling in startup code x86/startup_64: Drop global variables keeping track of LA57 state x86/startup_64: Simplify virtual switch on primary boot x86/head64: Replace pointer fixups with PIE codegen x86/head64: Simplify GDT/IDT initialization code asm-generic: Add special .pi.text section for position independent code x86: Move return_thunk to __pitext section x86/head64: Move early startup code into __pitext modpost: Warn about calls from __pitext into other text sections x86/coco: Make cc_set_mask() static inline x86/sev: Make all code reachable from 1:1 mapping __pitext x86/sev: Avoid WARN() in early code x86/sev: Use PIC codegen for early SEV startup code x86/sev: Drop inline asm LEA instructions for RIP-relative references x86/startup_64: Don't bother setting up GS before the kernel is mapped arch/x86/Makefile | 8 + arch/x86/boot/compressed/Makefile | 2 +- arch/x86/boot/compressed/misc.c | 22 +++ arch/x86/boot/compressed/pgtable_64.c | 2 - arch/x86/boot/compressed/sev.c | 6 + arch/x86/coco/core.c | 7 +- arch/x86/include/asm/coco.h | 8 +- arch/x86/include/asm/desc.h | 3 +- arch/x86/include/asm/init.h | 2 - arch/x86/include/asm/mem_encrypt.h | 8 +- arch/x86/include/asm/pgtable_64.h | 12 +- arch/x86/include/asm/pgtable_64_types.h | 15 +- arch/x86/include/asm/setup.h | 4 +- arch/x86/include/asm/sev.h | 6 +- arch/x86/include/uapi/asm/bootparam.h | 2 + arch/x86/kernel/Makefile | 7 + arch/x86/kernel/cpu/common.c | 2 - arch/x86/kernel/head64.c | 206 +++++++------------- arch/x86/kernel/head_64.S | 156 +++++---------- arch/x86/kernel/sev-shared.c | 54 +++-- arch/x86/kernel/sev.c | 27 ++- arch/x86/kernel/vmlinux.lds.S | 3 +- arch/x86/lib/Makefile | 13 -- arch/x86/lib/memcpy_64.S | 3 +- arch/x86/lib/memset_64.S | 3 +- arch/x86/lib/retpoline.S | 2 +- arch/x86/mm/Makefile | 2 +- arch/x86/mm/kasan_init_64.c | 3 - arch/x86/mm/mem_encrypt_boot.S | 3 +- arch/x86/mm/mem_encrypt_identity.c | 98 +++------- drivers/firmware/efi/libstub/efi-stub-helper.c | 8 + drivers/firmware/efi/libstub/efistub.h | 2 +- drivers/firmware/efi/libstub/x86-stub.c | 6 + include/asm-generic/vmlinux.lds.h | 3 + include/linux/init.h | 12 ++ scripts/mod/modpost.c | 11 +- tools/objtool/check.c | 26 +-- 37 files changed, 319 insertions(+), 438 deletions(-) base-commit: aa8eff72842021f52600392b245fb82d113afa8a -- 2.43.0.429.g432eaa2c6b-goog
