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 <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/d17655777c21bc09a7af1bbcf74e6f2b69a51152.1561610354.git.luto@kernel.org

1 files changed, 6 insertions, 1 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0082d1e56999..be8c3a680afa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5100,7 +5100,12 @@
 			targets for exploits that can control RIP.
 
 			emulate     [default] Vsyscalls turn into traps and are
-			            emulated reasonably safely.
+			            emulated reasonably safely.  The vsyscall
+				    page is readable.
+
+			xonly       Vsyscalls turn into traps and are
+			            emulated reasonably safely.  The vsyscall
+				    page is not readable.
 
 			none        Vsyscalls don't work at all.  This makes
 			            them quite hard to use for exploits but