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I was writing a section of my paper ("Designing a Secure and Friendly
Operating System") and basically describing and explaining why the
memory protection policy ("mprotect() restrictions") supplied by PaX is
a powerful security tool; and I had a thought. PaX places the policy in
the kernel; but with LSM hooks in mprotect() and friends, it may be
possible to implement it entirely in policy. This would be interesting,
as it would allow a system implementing the suggested enhancements to
avoid additional kernel code.
What I want to know is, what facilities does SELinux supply in the
current policy format to control the abuse of mprotect(), mmap(), and
friends; and where can I find an online reference on this?
For reference, PaX defines the following "good" mapping states where new
code can't be introduced (note that VM_READ and VM_MAYREAD are
completely ignored as they are of no consequence):
VM_WRITE
VM_MAYWRITE
VM_WRITE | VM_MAYWRITE
VM_EXEC
VM_MAYEXEC
VM_EXEC | VM_MAYEXEC
Of course the kernel won't allow VM_WRITE or VM_EXEC without VM_MAYWRITE
or VM_MAYEXEC, so this leaves us with
VM_MAYWRITE
VM_MAYEXEC
VM_WRITE | VM_MAYWRITE
VM_EXEC | VM_MAYEXEC
This gives us a few target guarantees:
1. No mapping may be created in any state other than those listed above
(VM_READ and VM_READ|VM_MAYREAD are permissible in addition to any
allowed state)
2. No mapping may transition to any state not described in (1)
3. No existing mapping without VM_EXEC is to have VM_MAYEXEC or be
given VM_EXEC or VM_MAYEXEC at any time in the remainder of its life
cycle; this includes mappings that started with and later dropped VM_EXEC
And there are a few other things that I want guaranteed:
1. Anonymous mappings are always created without VM_EXEC or VM_MAYEXEC
2. Shared memory mappings are always created without VM_EXEC or
VM_MAYEXEC (this is the current case)
3. File backed mmap() segments requesting PROT_WRITE get
VM_WRITE|VM_MAYWRITE (and VM_READ|VM_MAYREAD if PROT_READ was
requested), but never VM_EXEC or VM_MAYEXEC even if requested
4. For certain applications, it may be necessary that we can
automatically grant VM_EXEC|VM_MAYEXEC on file-backed mappings not
requesting PROT_WRITE, if those applications assume that PROT_READ
implies PROT_EXEC (this is how PaX works)
Finally, the ability to detect if the affected area is part of a
relocation and account for that in policy would be important, because
PaX set to disable ELF text relocations breaks quite a number of things;
trying to reconstruct the memory policy from PaX with an SELinux policy
without being able to mimic the special case allowance of ELF text
relocations would be disasterous.
REFERENCES:
http://pax.grsecurity.net/docs/mprotect.txt
- --
All content of all messages exchanged herein are left in the
Public Domain, unless otherwise explicitly stated.
Creative brains are a valuable, limited resource. They shouldn't be
wasted on re-inventing the wheel when there are so many fascinating
new problems waiting out there.
-- Eric Steven Raymond
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