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

From: Will Deacon <will@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Anders Roxell <anders.roxell@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	Andre Przywara <andre.przywara@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Dave P Martin <dave.martin@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64/alternatives: use subsections for replacement sequences
Date: Thu, 9 Jul 2020 13:43:42 +0100	[thread overview]
Message-ID: <20200709124342.GA28772@willie-the-truck> (raw)
In-Reply-To: <CAMj1kXFg5UN-JvPR3p5PVO015FdRwmDZE46qDvL-QJG-QC-oTg@mail.gmail.com>

On Thu, Jul 09, 2020 at 03:39:53PM +0300, Ard Biesheuvel wrote:
> On Thu, 9 Jul 2020 at 15:31, Ard Biesheuvel <ardb@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

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