Re: [PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early_pgt

[Alexander Potapenko]

On Wed, Aug 16, 2017 at 9:08 PM, Alexander Potapenko <glider@xxxxxxxxxx> 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.shutemov@xxxxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Michael Davidson <md@xxxxxxxxxx>
> Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
>  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

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