On Wed, Aug 16, 2017 at 9:08 PM, Alexander Potapenko <gli...@google.com> wrote:
> __startup_64() is normally using fixup_pointer() to access globals in a
> position-independent fashion. However |next_early_pgt| was accessed
> directly, which wasn't guaranteed to work.
>
> Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
> |next_early_pgt|, but Clang emitted a R_X86_64_32S, which led to
> accessing invalid memory and rebooting the kernel.
>
> Cc: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
> Cc: Dmitry Vyukov <dvyu...@google.com>
> Cc: Michael Davidson <m...@google.com>
> Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")
> Signed-off-by: Alexander Potapenko <gli...@google.com>
> ---
> arch/x86/kernel/head64.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 46c3c73e7f43..9ba79543d9ee 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -53,6 +53,7 @@ void __head __startup_64(unsigned long physaddr)
> pudval_t *pud;
> pmdval_t *pmd, pmd_entry;
> int i;
> + unsigned int *next_pgt_ptr;
>
> /* Is the address too large? */
> if (physaddr >> MAX_PHYSMEM_BITS)
> @@ -91,9 +92,9 @@ void __head __startup_64(unsigned long physaddr)
> * creates a bunch of nonsense entries but that is fine --
> * it avoids problems around wraparound.
> */
> -
> - pud = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
> - pmd = fixup_pointer(early_dynamic_pgts[next_early_pgt++], physaddr);
> + next_pgt_ptr = fixup_pointer(&next_early_pgt, physaddr);
> + pud = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
> + pmd = fixup_pointer(early_dynamic_pgts[(*next_pgt_ptr)++], physaddr);
>
> if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
> p4d = fixup_pointer(early_dynamic_pgts[next_early_pgt++],
> physaddr);
> --
> 2.14.1.480.gb18f417b89-goog
>
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
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Sitz der Gesellschaft: Hamburg
