linux-kernel - Re: [PATCH 21/35] arm64/gcs: Implement shadow stack prctl() interface

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Date:   Tue, 18 Jul 2023 17:51:54 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "corbet@....net" <corbet@....net>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "maz@...nel.org" <maz@...nel.org>,
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CC:     "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 21/35] arm64/gcs: Implement shadow stack prctl() interface

On Sun, 2023-07-16 at 22:51 +0100, Mark Brown wrote:
> +int arch_set_shadow_stack_status(struct task_struct *task, unsigned
> long arg)
> +{
> +       unsigned long gcs, size;
> +
> +       if (!system_supports_gcs())
> +               return -EINVAL;
> +
> +       if (is_compat_thread(task_thread_info(task)))
> +               return -EINVAL;
> +
> +       /* Reject unknown flags */
> +       if (arg & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK)
> +               return -EINVAL;
> +
> +       /* If the task has been locked block any attempted changes */
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_LOCK)
> +               return -EBUSY;
> +
> +       /* Drop flags other than lock if disabling */
> +       if (!(arg & PR_SHADOW_STACK_ENABLE))
> +               arg &= ~PR_SHADOW_STACK_LOCK;
> +
> +       /* If we are enabling GCS then make sure we have a stack */
> +       if (arg & PR_SHADOW_STACK_ENABLE) {
> +               if (!task_gcs_el0_enabled(task)) {
> +                       /* Do not allow GCS to be reenabled */
> +                       if (task->thread.gcs_base)
> +                               return -EINVAL;
> +
> +                       size = gcs_size(0);
> +                       gcs = alloc_gcs(task->thread.gcspr_el0, size,
> +                                       0, 0);
> +                       if (!gcs)
> +                               return -ENOMEM;
> +
> +                       task->thread.gcspr_el0 = gcs + size -
> sizeof(u64);
> +                       task->thread.gcs_base = gcs;
> +                       task->thread.gcs_size = size;
> +                       if (task == current)
> +                               write_sysreg_s(task-
> >thread.gcspr_el0,
> +                                              SYS_GCSPR_EL0);
> +
> +               }
> +       }
> +
> +       task->thread.gcs_el0_mode = arg;
> +       if (task == current)
> +               gcs_set_el0_mode(task);
> +

Ah! So does this task == current part mean this can be called from
another task via ptrace?

If so, then is the alloc_gcs() part on the wrong mm?