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Date: Tue, 23 Aug 2011 18:07:40 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Richard Weinberger <richard@nod.at>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@amd64.org>, Andrew Lutomirski <luto@mit.edu>,
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Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)
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On Tue, Aug 23, 2011 at 06:58:18PM +0200, Richard Weinberger wrote:

> What about this hack/solution?
> While booting UML can check whether the host's vDSO contains
> a SYSCALL instruction.
> If so, UML will not make the host's vDSO available to it's
> processes...

Note that this is *only* for 32bit side of things.  64bit one works fine...

I wouldn't search for SYSCALL in vdso, BTW - not when we have a good way
to trigger that crap and recognize it.

At boot time, fork a child.  Have it traced with PTRACE_SYSCALL.  Let it
put recognizable values in registers and call __kernel_vsyscall().  Then
let the parent do one more PTRACE_SYSCALL, then PTRACE_POKEUSER and set ebp
to 0x69696969.  PTRACE_CONT the sucker and let it report what it sees in ecx.
If it's what we'd put there - fine, it looks safe.  If it's 0x69696969 -
we have a problem, no vdso for us.