[PATCH] arm64/alternatives: use subsections for replacement sequences

[Will Deacon]

On Thu, Jul 09, 2020 at 03:39:53PM +0300, Ard Biesheuvel wrote:
> On Thu, 9 Jul 2020 at 15:31, Ard Biesheuvel <ardb at kernel.org> wrote:
> > It appears that the following code in alternatives.c
> >
> > static bool branch_insn_requires_update(struct alt_instr *alt, unsigned long pc)
> > {
> >     unsigned long replptr;
> >
> >     if (kernel_text_address(pc))
> >        return true;
> >
> > returns true inadvertently for the branch in this piece of code in entry.S
> >
> > alternative_if ARM64_HAS_IRQ_PRIO_MASKING
> >     ldr x20, [sp, #S_PMR_SAVE]
> >     msr_s SYS_ICC_PMR_EL1, x20
> >     mrs_s x21, SYS_ICC_CTLR_EL1
> >     tbz x21, #6, .L__skip_pmr_sync\@ // Check for ICC_CTLR_EL1.PMHE
> >     dsb sy // Ensure priority change is seen by redistributor
> > .L__skip_pmr_sync\@:
> >
> >
> > due to the fact that kernel_text_address() has no way of
> > distinguishing branches inside the subsection from branches that
> > require updating. So the alternatives patching code dutifully updates
> > the tbz opcode and points it to its original target in the subsection.
> >
> > This is going to be rather tricky to fix, unless we special case
> > tbz/cbz branches and other branches with limited range that would
> > never have worked before anyway.
> >
> > For now, better to just revert it and revisit it later.
> >
> 
> ... unless we decide to fix up all branches pointing outside the
> replacement sequence, which is not an entirely unreasonable thing to
> do:
> 
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index d1757ef1b1e7..7c205f9202a3 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -45,18 +45,11 @@
>  {
>         unsigned long replptr;
> 
> -       if (kernel_text_address(pc))
> -               return true;
> -
>         replptr = (unsigned long)ALT_REPL_PTR(alt);
>         if (pc >= replptr && pc <= (replptr + alt->alt_len))
>                 return false;
> 
> -       /*
> -        * Branching into *another* alternate sequence is doomed, and
> -        * we're not even trying to fix it up.
> -        */
> -       BUG();
> +       return true;

That looks better than the revert to me. Alex -- can you give it a spin with
your setup, please?

Will