Re: Pondering per-process vsyscall disablement

Marian Marinov
2014-05-23T02:44:35Z

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On 05/23/2014 02:04 AM, Andy Lutomirski wrote:
> It would be nice to have a way for new programs to declare that they don't need vsyscalls. What's the right way to
> do this? An ELF header entry in the loader? An ELF header entry in the program? A new arch_prctl?
>
> As background, there's an old part of the x86_64 ABI that allows programs to do gettimeofday, clock_gettime, and
> getcpu by calling to fixed addresses of the form 0xffffffffff600n00 where n indicates which of those three syscalls
> is being invoked. This is a security issue.
>
> Since Linux 3.1, vsyscalls are emulated using NX and page faults. As a result, vsyscalls no longer offer any
> performance advantage over normal syscalls; in fact, they're much slower. As far as I know, nothing newer than
> 2012 will attempt to use vsyscalls if a vdso is present. (Sadly, a lot of things will still fall back to the
> vsyscall page if there is no vdso, but that shouldn't matter, since there is always a vdso.)
>
> Despite the emulation, they could still be used as a weird form of ROP gadget that lives at a fixed address. I'd
> like to offer a way for new runtimes to indicate that they don't use vsyscalls so that the kernel can selectively
> disable emulation and remove the fixed-address executable code issue.
>
>
Wouldn't it be more useful if the check is against a bitmask added as extended attribute for that executable?
This way the administrators and will have the flexibility to simply add the new attribute to the executable.

Marian

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- --
Marian Marinov
Founder & CEO of 1H Ltd.
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