Date: Mon, 20 Apr 2015 11:07:05 -0400 From: John Baldwin <jhb@freebsd.org> To: Yue Chen <ychen.contact@gmail.com> Cc: freebsd-arch@freebsd.org Subject: Re: Situations about PC values in kernel data segments Message-ID: <2404384.sKCn9g0TDD@ralph.baldwin.cx> In-Reply-To: <CAKtBrB5Md0o-ERVBkLii11SROWXZd7HXUt=9Kk_verANb%2BKPVw@mail.gmail.com> References: <CAKtBrB6g5fR_tvT=KwrER4_VGfYB-fF-2DWmm1vMDpZ55qb2qg@mail.gmail.com> <6048769.xVxqkDkTGK@ralph.baldwin.cx> <CAKtBrB5Md0o-ERVBkLii11SROWXZd7HXUt=9Kk_verANb%2BKPVw@mail.gmail.com>
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On Friday, April 17, 2015 04:19:54 PM Yue Chen wrote:
> I mean, the PC values in non-.text segments like .data, .rodata, stack,
> heap, etc. Usually this is for comparison purposes. E.g., compare the
> faulting PC against some range already stored in a table/handler.
>
> > When pcb_onfault is used it is set to point to code in a .text segment,
> not anywhere else.
>
> The pointer value stored in non-.text segments is a PC value (instruction
> address in .text), like 0xffffffff12345678, and may not be a function entry
> point address, right?
I think I do not follow your question.
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. AFAIK, the kernel is not going to execute any code
from .data, .rodata, or the stack. For things like pcb_onfault, the value
stored is in .text, like this:
ENTRY(copyout)
PUSH_FRAME_POINTER
movq PCPU(CURPCB),%rax
movq $copyout_fault,PCB_ONFAULT(%rax)
testq %rdx,%rdx /* anything to do? */
jz done_copyout
...
done_copyout:
xorl %eax,%eax
movq PCPU(CURPCB),%rdx
movq %rax,PCB_ONFAULT(%rdx)
POP_FRAME_POINTER
ret
ALIGN_TEXT
copyout_fault:
movq PCPU(CURPCB),%rdx
movq $0,PCB_ONFAULT(%rdx)
movq $EFAULT,%rax
POP_FRAME_POINTER
ret
END(copyout)
Here 'copyout_fault' is in .text, not in a different section.
>
>
> On Fri, Apr 17, 2015 at 9:22 AM, John Baldwin <jhb@freebsd.org> 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.
> >
> > --
> > John Baldwin
> >
--
John Baldwin
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