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Date:      Fri, 17 Apr 2015 16:19:54 -0400
From:      Yue Chen <ychen.contact@gmail.com>
To:        John Baldwin <jhb@freebsd.org>
Cc:        freebsd-arch@freebsd.org
Subject:   Re: Situations about PC values in kernel data segments
Message-ID:  <CAKtBrB5Md0o-ERVBkLii11SROWXZd7HXUt=9Kk_verANb%2BKPVw@mail.gmail.com>
In-Reply-To: <6048769.xVxqkDkTGK@ralph.baldwin.cx>
References:  <CAKtBrB6g5fR_tvT=KwrER4_VGfYB-fF-2DWmm1vMDpZ55qb2qg@mail.gmail.com> <6048769.xVxqkDkTGK@ralph.baldwin.cx>

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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?


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
>



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