Re: [PATCH v2] KVM: arm64: Add missing BTI instructions

From: Mostafa Saleh <smostafa@google.com>
To: Sudeep Holla <sudeep.holla@arm.com>
Cc: maz@kernel.org, oliver.upton@linux.dev,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org, tabba@google.com,
	qperret@google.com, will@kernel.org, catalin.marinas@arm.com,
	yuzenghui@huawei.com, suzuki.poulose@arm.com,
	james.morse@arm.com, bgardon@google.com, gshan@redhat.com
Subject: Re: [PATCH v2] KVM: arm64: Add missing BTI instructions
Date: Fri, 7 Jul 2023 10:59:31 +0000	[thread overview]
Message-ID: <ZKfwE2VlaxXDoh8R@google.com> (raw)
In-Reply-To: <20230706162308.kyeitspgfaqb6vgn@bogus>

On Thu, Jul 06, 2023 at 05:23:08PM +0100, Sudeep Holla wrote:
> On Thu, Jul 06, 2023 at 03:22:40PM +0000, Mostafa Saleh wrote:
> > Some bti instructions were missing from
> > commit b53d4a272349 ("KVM: arm64: Use BTI for nvhe")
> > 
> > 1) kvm_host_psci_cpu_entry
> > kvm_host_psci_cpu_entry is called from __kvm_hyp_init_cpu through "br"
> > instruction as __kvm_hyp_init_cpu resides in idmap section while
> > kvm_host_psci_cpu_entry is in hyp .text so the offset is larger than
> > 128MB range covered by "b".
> > Which means that this function should start with "bti j" instruction.
> > 
> > LLVM which is the only compiler supporting BTI for Linux, adds "bti j"
> > for jump tables or by when taking the address of the block [1].
> > Same behaviour is observed with GCC.
> > 
> > As kvm_host_psci_cpu_entry is a C function, this must be done in
> > assembly.
> > 
> > Another solution is to use X16/X17 with "br", as according to ARM
> > ARM DDI0487I.a RLJHCL/IGMGRS, PACIASP has an implicit branch
> > target identification instruction that is compatible with
> > PSTATE.BTYPE 0b01 which includes "br X16/X17"
> > And the kvm_host_psci_cpu_entry has PACIASP as it is an external
> > function.
> > Although, using explicit "bti" makes it more clear than relying on
> > which register is used.
> > 
> > A third solution is to clear SCTLR_EL2.BT, which would make PACIASP
> > compatible PSTATE.BTYPE 0b11 ("br" to other registers).
> > However this deviates from the kernel behaviour (in bti_enable()).
> > 
> > 2) Spectre vector table
> > "br" instructions are generated at runtime for the vector table
> > (__bp_harden_hyp_vecs).
> > These branches would land on vectors in __kvm_hyp_vector at offset 8.
> > As all the macros are defined with valid_vect/invalid_vect, it is
> > sufficient to add "bti j" at the correct offset.
> > 
> > [1] https://reviews.llvm.org/D52867
> > 
> > Fixes: b53d4a272349 ("KVM: arm64: Use BTI for nvhe")
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > Reported-by: Sudeep Holla <sudeep.holla@arm.com>
> 
> Nothing change w.r.t cpu suspend-resume path in v2 anyways, but I assure
> I tested this again just be absolutely sure and it still fixes the issue
> I reported 😄, so
> 
> Tested-by: Sudeep Holla <sudeep.holla@arm.com>

Thanks for testing the patch again!