[PATCH v7 18/29] arm64: mte: Allow user control of the tag check mode via prctl()
Kevin Brodsky
kevin.brodsky at arm.com
Thu Jul 23 15:33:50 EDT 2020
On 20/07/2020 18:00, Dave Martin wrote:
> 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 at arm.com>
>>> Cc: Will Deacon <will at kernel.org>
>>> ---
>>>
>>> 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?
Isn't it as simple as checking if PF_KTHREAD is set in tsk->flags? At least this is
what ssbs_thread_switch() does.
Kevin
> 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
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