In order to be able to have tight control over which code may execute
from the early 1:1 mapping of memory, but still link vmlinux as a single
executable, prefix all symbol references in startup code with __pi_, and
invoke it from outside using the __pi_ prefix.
Use objtool to check that no absolute symbol references are present in
the startup code, as these cannot be used from code running from the 1:1
mapping.
Note that this also requires disabling the latent-entropy GCC plugin, as
the global symbol references that it injects would require explicit
exports, and given that the startup code rarely executes more than once,
it is not a useful source of entropy anyway.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-43-ardb+git@google.com
Invoke objtool on each startup code object individually to check for the
absence of absolute relocations. This is needed because this code will
be invoked from the 1:1 mapping of memory before those absolute virtual
addresses (which are derived from the kernel virtual base address
provided to the linker and possibly shifted at boot) are mapped.
Only objects built under arch/x86/boot/startup/ have this restriction,
and once they have been incorporated into vmlinux.o, this distinction is
difficult to make. So force the invocation of objtool for each object
file individually, even if objtool is deferred to vmlinux.o for the rest
of the build. In the latter case, only pass --noabs and nothing else;
otherwise, append it to the existing objtool command line.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-40-ardb+git@google.com
objtool already struggles to identify jump tables correctly in non-PIC
code, where the idiom is something like
jmpq *table(,%idx,8)
and the table is a list of absolute addresses of jump targets.
When using -fPIC, both the table reference as well as the jump targets
are emitted in a RIP-relative manner, resulting in something like
leaq table(%rip), %tbl
movslq (%tbl,%idx,4), %offset
addq %offset, %tbl
jmpq *%tbl
and the table is a list of offsets of the jump targets relative to the
start of the entire table.
Considering that this sequence of instructions can be interleaved with
other instructions that have nothing to do with the jump table in
question, it is extremely difficult to infer the control flow by
deriving the jump targets from the indirect jump, the location of the
table and the relative offsets it contains.
So let's not bother and disable jump tables for code built with -fPIC
under arch/x86/boot/startup.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-efi@vger.kernel.org
Link: https://lore.kernel.org/r/20250422210510.600354-2-ardb+git@google.com
When building with CONFIG_LTO_CLANG, there is an error in the x86 boot
startup code because it builds with a different code model than the rest
of the kernel:
ld.lld: error: Function Import: link error: linking module flags 'Code Model': IDs have conflicting values: 'i32 2' from vmlinux.a(head64.o at 1302448), and 'i32 1' from vmlinux.a(map_kernel.o at 1314208)
ld.lld: error: Function Import: link error: linking module flags 'Code Model': IDs have conflicting values: 'i32 2' from vmlinux.a(common.o at 1306108), and 'i32 1' from vmlinux.a(gdt_idt.o at 1314148)
As this directory is for code that only runs during early system
initialization, LTO is not very important, so filter out the LTO flags
from KBUILD_CFLAGS for arch/x86/boot/startup to resolve the build error.
Fixes: 4cecebf200 ("x86/boot: Move the early GDT/IDT setup code into startup/")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20250414-x86-boot-startup-lto-error-v1-1-7c8bed7c131c@kernel.org
Closes: https://lore.kernel.org/CA+G9fYvnun+bhYgtt425LWxzOmj+8Jf3ruKeYxQSx-F6U7aisg@mail.gmail.com/
The 5-level paging trampoline is used by both the EFI stub and the
traditional decompressor. Move it out of the decompressor sources into
the newly minted arch/x86/boot/startup/ sub-directory which will hold
startup code that may be shared between the decompressor, the EFI stub
and the kernel proper, and needs to tolerate being called during early
boot, before the kernel virtual mapping has been created.
This will allow the 5-level paging trampoline to be used by EFI boot
images such as zboot that omit the traditional decompressor entirely.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250401133416.1436741-10-ardb+git@google.com
