Re: [PATCH] arm64: Fix compile error with KVM and !HARDEN_BRANCH_PREDICTOR

[Marc Zyngier <maz@kernel.org>]

On 2019-11-29 07:25, Ard Biesheuvel wrote:
> On Fri, 29 Nov 2019 at 08:21, Marc Zyngier <maz@kernel.org> wrote:
>>
>> On Thu, 28 Nov 2019 17:20:20 +0000,
>>
>> [fixing Will's email address]
>>
>> Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>> >
>> > (+ Marc)
>> >
>> >
>> >
>> > On Wed, 27 Nov 2019 at 19:10, <minyard@acm.org> wrote:
>> > >
>> > > From: Corey Minyard <cminyard@mvista.com>
>> > >
>> > > When compiling with KVM enabled and without 
>> HARDEN_BRANCH_PREDICTOR,
>> > > the following compile error happens:
>> > >
>> > > arch/arm64/kernel/cpu_errata.c:92:23:
>> > > error: '__bp_harden_hyp_vecs_start' undeclared (first use in 
>> this function);
>> > > did you mean 'hyp_vecs_start'?
>> > >   void *dst = lm_alias(__bp_harden_hyp_vecs_start + slot * 
>> SZ_2K);
>> > >
>> > > Some ifdefs were removed by 3e91f3eacc91d9 "arm64: Always enable
>> > > spectre-v2 vulnerability detection" for 
>> CONFIG_HARDEN_BRANCH_PREDICTOR,
>> > > but __bp_harden_hyp_vecs_start is only defined if that config is
>> > > enabled.
>> > >
>> > > Add CONFIG_HARDEN_BRANCH_PREDICTOR to the #if that has 
>> CONFIG_KVM,
>> > > It looks like you need both of those for that code to be valid.
>> > >
>> > > Fixes: 3e91f3eacc91d9 "arm64: Always enable spectre-v2 
>> vulnerability detection"
>> > > Cc: Andre Przywara <andre.przywara@arm.com>
>> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
>> > > Cc: Stefan Wahren <stefan.wahren@i2se.com>
>> > > Cc: Will Deacon <will.deacon@arm.com>
>> > > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> > > Signed-off-by: Corey Minyard <cminyard@mvista.com>
>> > > ---
>> > > This is for 4.14, I'm not sure if it is needed for other 
>> kernels.
>> > >
>> > > It is not needed in master because a new config item was added,
>> > > CONFIG_KVM_INDIRECT_VECTORS, that depends on KVM and
>> > > HARDEN_BRANCH_PREDICTOR being configured.  I looked at pulling 
>> the
>> > > patches that add the required changes, and they make a lot of
>> > > changes.  This change is the simple fix, but I'm not sure if we 
>> want to
>> > > pull all those other changes into 4.14 and whatever other 
>> kernels
>> > > are required.
>> > >
>> >
>> > I agree that backporting this cleanly is going to be problematic,
>> > since it pulls in the entire EL2 VA randomization feature with all 
>> its
>> > prerequisites.
>> >
>> > Backporting the following set could be done fairly cleanly, and 
>> fixes
>> > the problem at build time, but unfortunately, it causes a boot 
>> time
>> > crash (see below)
>> >
>> > 97eca4d2bfec (HEAD -> linux-4.14.y) arm64: Move the content of 
>> bpi.S
>> > to hyp-entry.S
>> > 514dd33114c6 arm64: Make BP hardening slot counter available
>> > d7ddf3ae9470 arm64; insn: Add encoder for the EXTR instruction
>> > c0b2c4e56e10 arm64: KVM: Introduce EL2 VA randomisation
>> > 56ab0a87c737 arm64: KVM: Dynamically compute the HYP VA mask
>> > d92c02628dfb arm64: KVM: Allow far branches from vector slots to 
>> the
>> > main vectors
>> > 7adec01c9674 arm64: cpufeatures: Drop the ARM64_HYP_OFFSET_LOW 
>> feature flag
>> > 1095e4fc3134 arm64: KVM: Move stashing of x0/x1 into the vector 
>> code itself
>> > bb2e1aceb423 arm64: KVM: Dynamically patch the kernel/hyp VA mask
>> > 6f0ccfc451be arm64: KVM: Reserve 4 additional instructions in the 
>> BPI template
>> > bf425ffee07a arm64: insn: Add encoder for bitwise operations using 
>> literals
>> > 41dda94d1a9f arm64: insn: Add N immediate encoding
>> > 3225668ebe00 arm64: KVM: Move BP hardening vectors into .hyp.text 
>> section
>> >
>> > Marc?
>>
>> You need at least these:
>>
>> 1bb32a44aea1 KVM: arm/arm64: Keep GICv2 HYP VAs in 
>> kvm_vgic_global_state
>> 44a497abd621 KVM: arm/arm64: Do not use kern_hyp_va() with 
>> kvm_vgic_global_state
>>
>
> Yeah, I only did a fairly mechanical backport based on the actual
> diffs depend on each other, but I did spot those two as possibly
> related.
>
>> But that's definitely not enough to fix the crash.
>>
>> >
>> > [    0.062126] CPU: All CPU(s) started at EL1
>> > [    0.063109] alternatives: patching kernel code
>> > [    0.064228] random: get_random_u64 called from
>> > compute_layout+0x94/0xe8 with crng_init=0
>> > [    0.066313] aarch64_insn_gen_add_sub_imm: invalid immediate 
>> encoding 1904640
>>
>> OK, that one is really bizarre. This value (tag_val) is supposed to 
>> be
>> a small value (only 12 significant bits out of 24 at any given 
>> time),
>> and it is not (0x1D1000). So somehow compute_instruction() is not
>> doing the right thing.
>>
>> Do you have a tree somewhere with this patches?
>>
>
> Sure, thanks for having a look.
>
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=arm64-v4.14-backport%2b%2b

OK, I tracked it down to this missing patch:

11d764079c9f arm64: insn: Allow ADD/SUB (immediate) with LSL #12

I haven't tryied to boot the whole thing as a host though, only
tested it as a guest.

> Another thing I found bizarre is that we actually run this code when
> all CPUs boot at EL1. Or is that intended?

It is so that I can debug the whole thing in a guest! ;-)

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel