Messages in this thread | | | Date | Fri, 2 Oct 2015 08:23:40 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 26/26] x86, pkeys: Documentation |
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* Andy Lutomirski <luto@amacapital.net> wrote:
> >> Assuming it boots up fine on a typical distro, i.e. assuming that there are no > >> surprises where PROT_READ && PROT_EXEC sections are accessed as data. > > > > I can't wait to find out what implicitly expects PROT_READ from > > PROT_EXEC mappings. :)
So what seems to happen is that there are no pure PROT_EXEC mappings in practice - they are only omnibus PROT_READ|PROT_EXEC mappings, an unknown proportion of which truly relies on PROT_READ:
$ for C in firefox ls perf libreoffice google-chrome Xorg xterm \ konsole; do echo; echo "# $C:"; strace -e trace=mmap -f $C -h 2>&1 | cut -d, -f3 | \ grep PROT | sort | uniq -c; done
# firefox: 13 PROT_READ 82 PROT_READ|PROT_EXEC 184 PROT_READ|PROT_WRITE 2 PROT_READ|PROT_WRITE|PROT_EXEC
# ls: 2 PROT_READ 7 PROT_READ|PROT_EXEC 17 PROT_READ|PROT_WRITE
# perf: 1 PROT_READ 20 PROT_READ|PROT_EXEC 44 PROT_READ|PROT_WRITE
# libreoffice: 2 PROT_NONE 87 PROT_READ 148 PROT_READ|PROT_EXEC 339 PROT_READ|PROT_WRITE
# google-chrome: 39 PROT_READ 121 PROT_READ|PROT_EXEC 345 PROT_READ|PROT_WRITE
# Xorg: 1 PROT_READ 22 PROT_READ|PROT_EXEC 39 PROT_READ|PROT_WRITE
# xterm: 1 PROT_READ 25 PROT_READ|PROT_EXEC 46 PROT_READ|PROT_WRITE
# konsole: 1 PROT_READ 101 PROT_READ|PROT_EXEC 175 PROT_READ|PROT_WRITE
So whatever kernel side method we come up with, it's not something that I expect to become production quality. "Proper" conversion to pkeys has to be driven from the user-space side.
That does not mean we can not try! :-)
> There's one annoying issue at least: > > mprotect_pkey(..., PROT_READ | PROT_EXEC, 0) sets protection key 0. > mprotect_pkey(..., PROT_EXEC, 0) maybe sets protection key 15 or > whatever we use for this. What does mprotect_pkey(..., PROT_EXEC, 0) > do? What if the caller actually wants key 0? What if some CPU vendor > some day implements --x for real?
That comes from the hardcoded "user-space has 4 bits to itself, not managed by the kernel" assumption in the whole design. So no layering between different user-space libraries using pkeys in a different fashion, no transparent kernel use of pkeys (such as it may be), etc.
I'm not sure it's _worth_ managing these 4 bits, but '16 separate keys' does seem to be to me above a certain resource threshold that should be more explicitly managed than telling user-space: "it's all yours!".
> 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.
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.
Thanks,
Ingo
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