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    SubjectRe: [PATCH 26/26] x86, pkeys: Documentation
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    On 10/01/2015 11:23 PM, Ingo Molnar wrote:
    >> > Also, how do we do mprotect_pkey and say "don't change the key"?
    > So if we start managing keys as a resource (i.e. alloc/free up to 16 of them), and
    > provide APIs for user-space to do all that, then user-space is not supposed to
    > touch keys it has not allocated for itself - just like it's not supposed to write
    > to fds it has not opened.

    I like that. It gives us at least a "soft" indicator to userspace about
    what keys it should or shouldn't be using.

    > Such an allocation method can still 'mess up', and if the kernel allocates a key
    > for its purposes it should not assume that user-space cannot change it, but at
    > least for non-buggy code there's no interaction and it would work out fine.

    Yeah. It also provides a clean interface so that future hardware could
    enforce enforce kernel "ownership" of a key which could protect against
    even buggy code.

    So, we add a pair of syscalls,

    unsigned long sys_alloc_pkey(unsigned long flags??)
    unsigned long sys_free_pkey(unsigned long pkey)

    keep the metadata in the mm, and then make sure that userspace allocated
    it before it is allowed to do an mprotect_pkey() with it.

    mprotect_pkey(add, flags, pkey)
    {
    if (!(mm->pkeys_allocated & (1 << pkey))
    return -EINVAL;
    }

    That should be pretty easy to implement. The only real overhead is the
    16 bits we need to keep in the mm somewhere.


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