Re: [PATCH v4] arm64: mte: move register initialization to C

Catalin Marinas
2022-09-12T20:04:21Z

On Fri, Sep 09, 2022 at 05:54:13PM -0700, Peter Collingbourne wrote:
> On Tue, Sep 6, 2022 at 5:36 PM Evgenii Stepanov <eugenis@google.com> wrote:
> > If FEAT_MTE2 is disabled via the arm64.nomte command line argument on a
> > CPU that claims to support FEAT_MTE2, the kernel will use Tagged Normal
> > in the MAIR. If we interpret arm64.nomte to mean that the CPU does not
> > in fact implement FEAT_MTE2, setting the system register like this may
> > lead to UNSPECIFIED behavior. Fix it by arranging for MAIR to be set
> > in the C function cpu_enable_mte which is called based on the sanitized
> > version of the system register.
> >
> > There is no need for the rest of the MTE-related system register
> > initialization to happen from assembly, with the exception of TCR_EL1,
> > which must be set to include at least TBI1 because the secondary CPUs
> > access KASan-allocated data structures early. Therefore, make the TCR_EL1
> > initialization unconditional and move the rest of the initialization to
> > cpu_enable_mte so that we no longer have a dependency on the unsanitized
> > ID register value.
>
> Moving the register initialization to C also fixes a bug where the
> kernel's zeroing of TFSR_EL1 has no practical effect when the kernel
> is started in VHE mode because the register is currently being zeroed
> prior to the kernel enabling the redirect of TFSR_EL2 to TFSR_EL1 when
> it enables VHE. As a result, without this patch it is possible to get
> a spurious KASAN error report if TFSR_EL2 is non-zero out of reset.

Oh, I think this is a side-effect of the nVHE patches. We added MTE in
5.10 and __cpu_setup() was called at EL2 if the kernel was entered at
EL2 - 3b714d24ef17 ("arm64: mte: CPU feature detection and initial
sysreg configuration"). When nVHE turned up in 5.12, this was changed to
to run __cpu_setup at EL1 and this only initialises TFSR_EL1.
__finalise_el2 should have transferred TFSR_EL12.

I don't think there other registers we missed in __cpu_setup() but I
haven't looked in detail.

So for this, we either move the reg initialisation to C or we fix
__finalise_el2. I'm tempted to go with the former as long as the kernel
doesn't read that register up to that point and complain of a spurious
asynchronous fault.

--
Catalin