Re: [PATCH 26/26] x86, pkeys: Documentation

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Dave Hansen <dave@sr71.net> wrote:
> 
> > > Another question, related to enumeration as well: I'm wondering whether 
> > > there's any way for the kernel to allocate a bit or two for its own purposes - 
> > > such as protecting crypto keys? Or is the facility fundamentally intended for 
> > > user-space use only?
> > 
> > No, that's not possible with the current setup.
> 
> Ok, then another question, have you considered the following usecase:

So, I'm wondering about the following additional usecase:

Right now the native x86 PTE format allows two protection related bits for 
user-space pages:

  _PAGE_BIT_RW:                   if 0 the page is read-only,  if 1 then it's read-write
  _PAGE_BIT_NX:                   if 0 the page is executable, if 1 then it's not executable

As discussed previously, pkeys allows 'true execute only (--x)' mappings.

Another possibility would be 'true write-only (-w-)' mappings.

This too could in theory be introduced 'transparently', via 'pure PROT_WRITE' 
mappings (i.e. no PROT_READ|PROT_EXEC bits set). Assuming the amount of user-space 
with implicit 'PROT_WRITE implies PROT_READ' assumptions is not unmanageble for a 
distro willing to try this.

Usage of this would be more limited than of pure PROT_EXEC mappings, but it's a 
nonzero set:

 - Write-only log buffers that are normally mmap()-ed from a file.

 - Write-only write() IO buffers that are only accessed via write().
   (kernel-space accesses ignore pkey values.)

   glibc's buffered IO might possibly make use of this, for write-only
   fopen()ed files.

 - Language runtimes could improve their security by eliminating W+X mappings of 
   JIT-ed code, instead they could use two alias mappings: one alias is a 
   true-exec (--x) mapping, the other (separately mapped, separately randomized)
   mapping is a true write-only (--x) mapping for generated code.

In addition to the security advantage, another advantage would be increased 
robustness: no accidental corruption of IO (or JIT) buffers via read-only 
codepaths.

Another advantage would be that it would utilize pkeys without having to teach 
applications to use new system calls.

Thanks,

	Ingo

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