Messages in this thread | | | Subject | Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX. | From | Dave Hansen <> | Date | Mon, 8 Jan 2018 09:28:28 -0800 |
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On 01/08/2018 09:05 AM, Thomas Gleixner wrote: > On Mon, 8 Jan 2018, Willy Tarreau wrote: >> Since we're going to keep running on the same PGD when returning to >> userspace for certain performance-critical tasks, we'll need the user >> pages to be executable. So this code disables the extra protection >> that was added consisting in marking user pages _PAGE_NX so that this >> pgd remains usable for userspace. >> >> Note: it isn't necessarily the best approach, but one way or another >> if we want to be able to return to userspace from the kernel, >> we'll have to have this executable anyway. Another approach >> might consist in using another pgd for userland+kernel but >> the current core really looks like an extra careful measure >> to catch early bugs if any. > > I surely want to keep that as a safety measure. The entry code is simple to > get wrong and running with the wrong pagetables by a silly mistake and > thereby undoing the protection is surely not what we want. > > Need to find a free time slot to think about that.
This does get immensely easier if we choose a mode at exec() (or fork() even) and never change it. The prctl() _could_ just be a flag to tell what your children should do.
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