Situations about PC values in kernel data segments

[Yue Chen]

> Are you asking if you can figure out if a given PC value used as the value
> of $rip for an arbitrary instruction is valid, or are you trying to
enumerate
> all the words in memory that hold a pointer to a .text value (like
> pcb_onfault)?
> I assumed the former.

So sorry for the confusion. I mean any other situations of the *latter*
one, which *excludes* function pointers.
And this does not have to be a full-word pointer. This can be a half-word
displacement/offset
to the address in .text, or a special encoding of the address as well.



On Mon, Apr 20, 2015 at 11:00 AM, John Baldwin <jhb at freebsd.org> wrote:

> On Friday, April 17, 2015 04:43:48 PM Konstantin Belousov wrote:
> > On Fri, Apr 17, 2015 at 09:22:43AM -0400, John Baldwin wrote:
> > > On Saturday, April 11, 2015 05:18:28 AM Yue Chen wrote:
> > > > Dear all,
> > > >
> > > > We are working on a project about OS security.
> > > > We wonder in which situations the program counter (PC) value (e.g.,
> the
> > > > value in %RIP on x86_64, i.e, instruction address) could be in kernel
> > > > (module) data segments (including stack, heap, etc.).
> > > >
> > > > Here we mainly care about the address/value that are NOT function
> entry
> > > > points since there exist a number of function pointers. Also, we only
> > > > consider the normal cases because one can write arbitrary values
> into a
> > > > variable/pointer. And we mainly consider i386, AMD64 and ARM.
> > > >
> > > > Here are some situations I can think about:
> > > > function/interrupt/exception/syscall return address on stack;
> switch/case
> > > > jump table target; page fault handler (pcb_onfault on *BSD);
> restartable
> > > > atomic sequences (RAS) registry; thread/process context structure
> like Task
> > > > state segment (TSS), process control block (PCB) and thread control
> block
> > > > (TCB); situations for debugging purposes (e.g., like those in
> ``segment not
> > > > present'' exception handler).
> > > >
> > > > Additionally, does any of these addresses have offset formats or
> special
> > > > encodings? For example, on x86_64, we may use 32-bit RIP-relative
> > > > (addressing) offset to represent a 64-bit full address. In glibc's
> > > > setjmp/longjmp jmp_buf, they use a special encoding (PTR_MANGLE) for
> saved
> > > > register values.
> > >
> > > For i386 and amd64, I think all of the code that is executed does live
> in a
> > > .text segment.  When pcb_onfault is used it is set to point to code in
> a .text
> > > segment, not anywhere else.  Similarly, fault and exception handlers
> as well
> > > as the stub for new threads/processes after fork/thread_create is in
> .text
> > > as well.  There are multiple text segments present when modules are
> loaded
> > > of course, but you should be able to enumerate all of those in the
> linker.
> >
> > Wasn't bpf enhanced to compile filters to the native code, on x86 ?
> > Also, what about BIOS code ? Esp. since the spread of UEFI and hope that
> > our kernel starts using UEFI runtime services one day.  My point is that
> > _relying_ on enumeration of the text segments for kernel and modules to
> > determine all executable memory is not correct.
>
> It depends on the scope.  If this is for a graduate research project to
> build
> a prototype to see if this is feasible, then some cavets are acceptable if
> they are known.  One could be to disallow the bpf JIT option (I believe it
> is
> not in GENERIC)?  EFI is actually fairly easily handled since the EFI
> memory
> map gives you the bounds of the executable code and you can just treat
> that as
> an additional .text segment.
>
> --
> John Baldwin
>