Date: Mon, 20 Apr 2015 13:54:28 -0400 From: Yue Chen <ychen.contact@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-arch@freebsd.org Subject: Re: Situations about PC values in kernel data segments Message-ID: <CAKtBrB6=qDbHyJizbuUDWcRMpTVQev8YeUEnQ3AUCC6%2BXor3HQ@mail.gmail.com> In-Reply-To: <2177000.nIlZYR4khO@ralph.baldwin.cx> References: <CAKtBrB6g5fR_tvT=KwrER4_VGfYB-fF-2DWmm1vMDpZ55qb2qg@mail.gmail.com> <6048769.xVxqkDkTGK@ralph.baldwin.cx> <20150417134348.GR2390@kib.kiev.ua> <2177000.nIlZYR4khO@ralph.baldwin.cx>
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> 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@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 >
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