Messages in this thread | | | Date | Wed, 30 Nov 2011 10:10:35 -0800 | Subject | Re: [rfc 3/3] prctl: Add PR_SET_MM codes to tune up mm_struct entires | From | Kees Cook <> |
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On Wed, Nov 30, 2011 at 9:37 AM, Cyrill Gorcunov <gorcunov@gmail.com> wrote: > On Tue, Nov 29, 2011 at 12:40:57PM -0800, Kees Cook wrote: >> > >> > On the other hands these fields are set up by elf hanlder code, which >> > does mmap these areas, so we have to check that particular member >> > belongs to existing VMA and never cross user-space area, and together >> > with root-only approach would not it be enough? I'm sure missing something >> > that is why I'm asking. >> >> Right, if you verify that the addresses are actually inside valid >> userspace vmas, that is likely to be right, though there are probably >> other things I haven't thought of. The trouble is avoiding vdso, stack >> guard page, vsyscall, and anything else that isn't meant for the mm to >> have direct access to. >> > > Hi Kees, > > what about this one? Note that these mm_struct members don't affect > kernel much (at least as far as I see, except maybe brk,start_brk and > start_stack values), so I've added some sanity checks here, hope they > would fit. Still main protection is root-only access only. The kernel > itself uses vma_area::start/end members for overlows tests internally > so I think even passing crazy data here won't crash the kernel itself.
Right, though besides just crashing the kernel, I'm trying to look at this from the perspective of a paranoid admin that doesn't even trust the root user. Is there some way this new interface could be used to provide the building blocks for gaining kernel execute control? (Imagine a system running with modules disabled, STRICT_DEVMEM enabled, etc.)
> What do you think?
This looks way better, yes. I have this feeling like these validations should be more centralized or tied to the mm code more directly to avoid drift, but I don't have any constructive suggestions unfortunately. Maybe other folks do?
> + switch (opt) { > + case PR_SET_MM_START_CODE: > + case PR_SET_MM_END_CODE: > + > + vm_req_flags = VM_READ | VM_EXEC; > + vm_bad_flags = VM_WRITE | VM_MAYSHARE; > + > + if ((vma->vm_flags & vm_req_flags) != vm_req_flags || > + (vma->vm_flags & vm_bad_flags)) > + goto out;
Another random thought: given this very regular set of checks you're doing, perhaps the flags should be part of a data structure instead, just to reduce the size of this routine?
struct mm_flags { int req_flags; int bad_flags; };
struct mm_flags opt_flags[] = { ... { VM_READ | VM_EXEC, VM_WRITE | VM_MAYSHARE }, /* PR_SET_MM_START_CODE */ { VM_READ | VM_EXEC, VM_WRITE | VM_MAYSHARE }, /* PR_SET_MM_END_CODE */ ...
then do validation before the switch statement all in one place, and leave the switch for more programmatic checks?
-Kees
-- Kees Cook ChromeOS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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