x86/vsyscall: Add a new vsyscall=xonly mode

With vsyscall emulation on, a readable vsyscall page is still exposed that
contains syscall instructions that validly implement the vsyscalls.

This is required because certain dynamic binary instrumentation tools
attempt to read the call targets of call instructions in the instrumented
code.  If the instrumented code uses vsyscalls, then the vsyscall page needs
to contain readable code.

Unfortunately, leaving readable memory at a deterministic address can be
used to help various ASLR bypasses, so some hardening value can be gained
by disallowing vsyscall reads.

Given how rarely the vsyscall page needs to be readable, add a mechanism to
make the vsyscall page be execute only.

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Kernel Hardening <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lkml.kernel.org/r/d17655777c21bc09a7af1bbcf74e6f2b69a51152.1561610354.git.luto@kernel.org

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