On Mon, Jul 20, 2020 at 04:30:35PM +0100, Kevin Brodsky wrote:
> On 15/07/2020 18:08, Catalin Marinas wrote:
> >By default, even if PROT_MTE is set on a memory range, there is no tag
> >check fault reporting (SIGSEGV). Introduce a set of option to the
> >exiting prctl(PR_SET_TAGGED_ADDR_CTRL) to allow user control of the tag
> >check fault mode:
> >
> > PR_MTE_TCF_NONE - no reporting (default)
> > PR_MTE_TCF_SYNC - synchronous tag check fault reporting
> > PR_MTE_TCF_ASYNC - asynchronous tag check fault reporting
> >
> >These options translate into the corresponding SCTLR_EL1.TCF0 bitfield,
> >context-switched by the kernel. Note that uaccess done by the kernel is
> >not checked and cannot be configured by the user.
> >
> >Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> >Cc: Will Deacon <will@xxxxxxxxxx>
> >---
> >
> >Notes:
> > v3:
> > - Use SCTLR_EL1_TCF0_NONE instead of 0 for consistency.
> > - Move mte_thread_switch() in this patch from an earlier one. In
> > addition, it is called after the dsb() in __switch_to() so that any
> > asynchronous tag check faults have been registered in the TFSR_EL1
> > registers (to be added with the in-kernel MTE support.
> > v2:
> > - Handle SCTLR_EL1_TCF0_NONE explicitly for consistency with PR_MTE_TCF_NONE.
> > - Fix SCTLR_EL1 register setting in flush_mte_state() (thanks to Peter
> > Collingbourne).
> > - Added ISB to update_sctlr_el1_tcf0() since, with the latest
> > architecture update/fix, the TCF0 field is used by the uaccess
> > routines.
[...]
> >diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
[...]
> >+void mte_thread_switch(struct task_struct *next)
> >+{
> >+ if (!system_supports_mte())
> >+ return;
> >+
> >+ /* avoid expensive SCTLR_EL1 accesses if no change */
> >+ if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
>
> I think this could be improved by checking whether `next` is a kernel
> thread, in which case thread.sctlr_tcf0 is 0 but there is no point in
> setting SCTLR_EL1.TCF0, since there should not be any access via TTBR0.
Out of interest, do we have a nice way of testing for a kernel thread
now?
I remember fpsimd_thread_switch() used to check for task->mm, but we
seem to have got rid of that at some point. set_mm() can defeat this,
and anyway the heavy lifting for FPSIMD is now deferred until returning
to userspace.
Cheers
---Dave
