Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 12 Dec 2014 17:45:03 -0800 | Subject | Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit kernels |
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On Fri, Dec 12, 2014 at 4:23 PM, Dave Hansen <dave@sr71.net> wrote: > On 12/12/2014 04:11 PM, Andy Lutomirski wrote: >> On Fri, Dec 12, 2014 at 3:16 PM, Dave Hansen <dave@sr71.net> wrote: >>> On 12/12/2014 03:04 PM, Andy Lutomirski wrote: >>>> Anyway, do your patches handle the case where a 32-bit app maliciously >>>> executes a 64-bit mpx insn with a very large address? I think it's >>>> okay, but I might have missed something. >>> >>> You mean in the instruction decoder? I haven't tried that case >>> explicitly, but I did do a substantial amount of testing throwing random >>> instruction streams at the decoder to make sure it never fell over. >>> (Well, mostly random, I made sure to throw the MPX opcodes in there a >>> bunch so it would get much deeper in to the decoder). >>> >>> It's not about the instruction size, it's about the mode the CPU is in. >>> If a 32-bit app manages to switch over to 64-bit mode and doesn't tell >>> the kernel (TIF_IA32 remains set), then we'll treat it as a 32-bit >>> instruction. >> >> The insn decoder should probably use user_64bit_mode, not TIF_IA32. >> It's actually quite easy to far jump/call/ret or sigreturn to a >> different bitness. > > There are number of examples of this in the kernel today: > > #ifdef CONFIG_X86_64 > is_64bit = kernel_ip(to) || !test_thread_flag(TIF_IA32); > #endif > insn_init(&insn, kaddr, size, is_64bit); > > Are you saying that those need to get fixed up? >
Yes, although so far it looks like the only real danger with them is that userspace could shoot itself in the foot.
>>> The kernel might end up going and looking for the bounds tables in some >>> funky places if the kernel and the hardware disagree about 32 vs. 64-bit >>> modes, but it's not going to do any harm since we treat all of the data >>> we get from MPX (instruction decoding, register contents, bounds table >>> contents, etc...) as completely untrusted. >>> >>> It's a nice, paranoid thing to ask and I'm glad you brought it up >>> because I hadn't thought about it, but I don't think any harm can come >>> of it. >> >> Paranoia is fun! >> >> The only thing I'd really be worried about is if the code that turns >> va into bounds table offset generates some absurdly large offset as a >> result and causes a problem. > > The instructions that get decoded have *NOTHING* to do with the mode > we're running in. By the time we take a bounds fault and copy the > instruction in from the instruction pointer, we have absolutely no idea > what was actually being executed, no matter what mode we are running in. > > I believe the instruction decoder already happily handles this. > > Furthermore, we don't even *USE* the result of the instruction decode in > the kernel. We toss it in to the siginfo and hand it out to userspace.
Hmm. I may have confused myself.
I was thinking of this:
+ if (is_64bit_mm(mm)) { + vaddr_space_size = 1ULL << __VIRTUAL_MASK_SHIFT; + bd_entry_virt_space = vaddr_space_size / MPX_BD_NR_ENTRIES_64; + /* + * __VIRTUAL_MASK takes the 64-bit addressing hole + * in to accout. This is a noop on 32-bit. + */ + addr &= __VIRTUAL_MASK; + return addr / bd_entry_virt_space; + } else { + vaddr_space_size = (1ULL << 32); + bd_entry_virt_space = vaddr_space_size / MPX_BD_NR_ENTRIES_32; + return addr / bd_entry_virt_space; + }
Is there a scenario in which the return value ends up being insanely high? If so, does it matter?
--Andy
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