Messages in this thread | | | Date | Mon, 29 Jun 2020 09:04:04 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v3 5/7] x86/boot: Remove run-time relocations from .head.text code |
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On Mon, Jun 29, 2020 at 10:09:26AM -0400, Arvind Sankar wrote: > The assembly code in head_{32,64}.S, while meant to be > position-independent, generates run-time relocations because it uses > instructions such as > leal gdt(%edx), %eax > which make the assembler and linker think that the code is using %edx as > an index into gdt, and hence gdt needs to be relocated to its run-time > address. > > On 32-bit, with lld Dmitry Golovin reports that this results in a > link-time error with default options (i.e. unless -z notext is > explicitly passed): > LD arch/x86/boot/compressed/vmlinux > ld.lld: error: can't create dynamic relocation R_386_32 against local > symbol in readonly segment; recompile object files with -fPIC or pass > '-Wl,-z,notext' to allow text relocations in the output > > With the BFD linker, this generates a warning during the build, if > --warn-shared-textrel is enabled, which at least Gentoo enables by > default: > LD arch/x86/boot/compressed/vmlinux > ld: arch/x86/boot/compressed/head_32.o: warning: relocation in read-only section `.head.text' > ld: warning: creating a DT_TEXTREL in object > > On 64-bit, it is not possible to link the kernel as -pie with lld, and > it is only possible with a BFD linker that supports -z noreloc-overflow, > i.e. versions >2.26. This is because these instructions cannot really be > relocated: the displacement field is only 32-bits wide, and thus cannot > be relocated for a 64-bit load address. The -z noreloc-overflow option > simply overrides the linker error, and results in R_X86_64_RELATIVE > relocations that apply a 64-bit relocation to a 32-bit field anyway. > This happens to work because nothing will process these run-time > relocations.
Are any of Thomas Garnier's PIE fixes useful here too? He had a lot of fixes to make changes for PC-relative addressing in the various assembly bits: https://lore.kernel.org/lkml/20200228000105.165012-9-thgarnie@chromium.org/
> > Start fixing this by removing relocations from .head.text: > - On 32-bit, use a base register that holds the address of the GOT and > reference symbol addresses using @GOTOFF, i.e. > leal gdt@GOTOFF(%edx), %eax > - On 64-bit, most of the code can (and already does) use %rip-relative > addressing, however the .code32 bits can't, and the 64-bit code also > needs to reference symbol addresses as they will be after moving the > compressed kernel to the end of the decompression buffer. > For these cases, reference the symbols as an offset to startup_32 to > avoid creating relocations, i.e. > leal (gdt-startup_32)(%bp), %eax > This only works in .head.text as the subtraction cannot be represented > as a PC-relative relocation unless startup_32 is in the same section > as the code. Move efi32_pe_entry into .head.text so that it can use > the same method to avoid relocations. > > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu> > Reviewed-by: Ard Biesheuvel <ardb@kernel.org> > Reviewed-by: Fangrui Song <maskray@google.com>
Nice.
Reviewed-by: Kees Cook <keescook@chromium.org>
-- Kees Cook
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