Situations about PC values in kernel data segments

[Konstantin Belousov]

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.