Re: [PATCH] x86/extable: ensure entries are swapped completely when sorting

[Ard Biesheuvel]

On 10 May 2016 at 23:07, Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
> The x86 exception table sorting was changed in commit 29934b0fb8ff
> ("x86/extable: use generic search and sort routines") to use the arch
> independent code in lib/extable.c. However, the patch was mangled
> somehow on its way into the kernel from the last version posted at [1].
> The committed version kind of attempted to incorporate the changes of
> commit 548acf19234d ("x86/mm: Expand the exception table logic to allow
> new handling options") as in _completely_ _ignoring_ the x86 specific
> 'handler' member of struct exception_table_entry. This effectively broke
> the sorting as entries will only partly be swapped now.
>

OK, it appears that Tony's patches grew a 'handler' field fairly late
in their review cycle, and I didn't pick up on that. Apologies.

> Fortunately, the x86 Kconfig selects BUILDTIME_EXTABLE_SORT, so the
> exception table doesn't need to be sorted at runtime. However, in case
> that ever changes, we better not break the exception table sorting just
> because of that.
>

BUILDTIME_EXTABLE_SORT applies to the core image only, but we still
rely on the sorting routines for modules in that case. So I think this
needs to be fixed right away.

> Fix this by providing a swap_ex_entry_fixup() macro that takes care of
> the 'handler' member.
>
> [1] https://lkml.org/lkml/2016/1/27/232
>
> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>

Fixes: 29934b0fb8f ("x86/extable: use generic search and sort routines")
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

Thanks,
Ard.

> ---
>  arch/x86/include/asm/uaccess.h |    8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index a969ae607be8..b69b7bffb0e0 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -108,6 +108,14 @@ struct exception_table_entry {
>
>  #define ARCH_HAS_RELATIVE_EXTABLE
>
> +#define swap_ex_entry_fixup(a, b, tmp, delta)                  \
> +       do {                                                    \
> +               (a)->fixup = (b)->fixup + (delta);              \
> +               (b)->fixup = (tmp).fixup - (delta);             \
> +               (a)->handler = (b)->handler + (delta);          \
> +               (b)->handler = (tmp).handler - (delta);         \
> +       } while (0)
> +
>  extern int fixup_exception(struct pt_regs *regs, int trapnr);
>  extern bool ex_has_fault_handler(unsigned long ip);
>  extern int early_fixup_exception(unsigned long *ip);
> --
> 1.7.10.4
>