linux-kernel - Re: [PATCH v2 01/11] arm64: use RET instruction for exiting the trampoline

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Date:   Mon, 8 Jan 2018 14:38:00 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>
Subject: Re: [PATCH v2 01/11] arm64: use RET instruction for exiting the trampoline

On 8 January 2018 at 14:33, Will Deacon <will.deacon@....com> wrote:
> On Sat, Jan 06, 2018 at 01:13:23PM +0000, Ard Biesheuvel wrote:
>> On 5 January 2018 at 13:12, Will Deacon <will.deacon@....com> wrote:
>> > Speculation attacks against the entry trampoline can potentially resteer
>> > the speculative instruction stream through the indirect branch and into
>> > arbitrary gadgets within the kernel.
>> >
>> > This patch defends against these attacks by forcing a misprediction
>> > through the return stack: a dummy BL instruction loads an entry into
>> > the stack, so that the predicted program flow of the subsequent RET
>> > instruction is to a branch-to-self instruction which is finally resolved
>> > as a branch to the kernel vectors with speculation suppressed.
>> >
>>
>> How safe is it to assume that every microarchitecture will behave as
>> expected here? Wouldn't it be safer in general not to rely on a memory
>> load for x30 in the first place? (see below) Or may the speculative
>> execution still branch anywhere even if the branch target is
>> guaranteed to be known by that time?
>
> The main problem with this approach is that EL0 can read out the text and
> find the kaslr offset.

Not really - the CONFIG_RANDOMIZE_BASE path puts the movz/movk
sequence in the next page, but that does involve an unconditional
branch.

> The memory load is fine, because the data page is
> unmapped along with the kernel text. I'm not aware of any
> micro-architectures where this patch doesn't do what we need.
>

Well, the memory load is what may incur the delay, creating the window
for speculative execution of the indirect branch. What I don't have
enough of a handle on is whether this speculative execution may still
branch to wherever the branch predictor is pointing even if the
register containing the branch target is already available.