Re: [PATCH v1 4/5] arm64: Inject UNDEF when accessing MTE sysregs with MTE disabled

[Marc Zyngier <maz@kernel.org>]

On Thu, 27 Nov 2025 14:41:24 +0000,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi Marc,
> 
> On Thu, 27 Nov 2025 at 14:17, Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Thu, 27 Nov 2025 12:22:09 +0000,
> > Fuad Tabba <tabba@google.com> wrote:
> > >
> > > When MTE hardware is present but disabled via software (arm64.nomte or
> > > CONFIG_ARM64_MTE=n), HCR_EL2.ATA is cleared to prevent use of MTE
> > > instructions. However, this alone doesn't fully emulate hardware that
> > > lacks MTE support.
> > >
> > > With HCR_EL2.ATA cleared, accesses to certain MTE system registers trap
> > > to EL2 with exception class ESR_ELx_EC_SYS64. To faithfully emulate
> > > hardware without MTE (where such accesses would cause an Undefined
> > > Instruction exception), inject UNDEF into the host.
> > >
> > > Signed-off-by: Fuad Tabba <tabba@google.com>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 44 ++++++++++++++++++++++++++++++
> > >  1 file changed, 44 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > index 29430c031095..f542e4c17156 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > @@ -686,6 +686,46 @@ static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
> > >       kvm_skip_host_instr();
> > >  }
> > >
> > > +static void inject_undef64(void)
> > > +{
> > > +     unsigned long sctlr, vbar, old, new;
> > > +     u64 offset, esr;
> > > +
> > > +     vbar = read_sysreg_el1(SYS_VBAR);
> > > +     sctlr = read_sysreg_el1(SYS_SCTLR);
> > > +     old = read_sysreg_el2(SYS_SPSR);
> > > +     new = get_except64_cpsr(old, system_supports_mte(), sctlr, PSR_MODE_EL1h);
> > > +     offset = get_except64_offset(old, PSR_MODE_EL1h, except_type_sync);
> > > +     esr = (ESR_ELx_EC_UNKNOWN << ESR_ELx_EC_SHIFT) | ESR_ELx_IL;
> > > +
> > > +     write_sysreg_el1(esr, SYS_ESR);
> > > +     write_sysreg_el1(read_sysreg_el2(SYS_ELR), SYS_ELR);
> > > +     write_sysreg_el1(old, SYS_SPSR);
> > > +     write_sysreg_el2(vbar + offset, SYS_ELR);
> > > +     write_sysreg_el2(new, SYS_SPSR);
> > > +}
> > > +
> > > +static bool handle_host_mte(u64 esr)
> > > +{
> > > +     /* If we're here for any reason other than MTE, then it's a bug. */
> > > +
> > > +     if (read_sysreg(HCR_EL2) & HCR_ATA)
> > > +             return false;
> > > +
> > > +     switch (esr_sys64_to_sysreg(esr)) {
> > > +     case SYS_RGSR_EL1:
> > > +     case SYS_GCR_EL1:
> > > +     case SYS_TFSR_EL1:
> > > +     case SYS_TFSRE0_EL1:
> >
> > How about other things, such as DC GVA? Don't you need to trap and
> > UNDEF it (which has the side effect of also trapping DC ZVA)?
> >
> > Same question for all the DC {C,I,CI}GVA{C,P} instructions.
> 
> As far as I could tell, none of these are trapped by ATA. The spec
> says that in the absence of MTE, their behavior is undefined --- which
> is the same for the ones I'm actually handling here...
> 
> The reasons I've only handled these is that, when booting a system
> with a misadvertised MTE, the kernel accesses these registers, and
> injecting an UNDEF resulted in a nicer failure mode.

But it all comes down to *why* is MTE disabled. Is it because the user
cannot be arsed with MTE's abysmal^Wstellar performance? Or because
this is a memory corruption vector on a misconfigured platform?

> What do you suggest, drop this patch (and the one before it), since
> trying to access MTE is UNDEF, and the kernel that does it is just
> shooting itself in the foot (no security implications)? Or edit the
> commit message to make it clear that this is best effort, based on
> what ATA traps?

No, what I am simply pointing out that that there is more to MTE than
what gets trapped by ATA, and my hunch is that when MTE is disabled on
machine that actually has it, it is because something is deeply broken
with tag management (I have one such machine).

So depending which side of the problem you're on, this could be either
perfectly valid, or just missing the point.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.