perf/x86/intel/lbr: Fix LBR filter

The LBR 'from' adddress is under full userspace control; ensure
we validate it before reading from it.

Note: is_module_text_address() can potentially be quite
expensive; for those running into that with high overhead
in modules optimize it using an RCU backed rb-tree.
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
Cc: eranian@google.com
Link: http://lkml.kernel.org/r/20130503121256.158211806@chello.nlSigned-off-by: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/n/tip-mk8i82ffzax01cnqo829iy1q@git.kernel.org

arch/x86/kernel/cpu/perf_event_intel_lbr.c

@@ -442,8 +442,18 @@ static int branch_type(unsigned long from, unsigned long to)
 			return X86_BR_NONE;
 
 		addr = buf;
-	} else
-		addr = (void *)from;
+	} else {
+		/*
+		 * The LBR logs any address in the IP, even if the IP just
+		 * faulted. This means userspace can control the from address.
+		 * Ensure we don't blindy read any address by validating it is
+		 * a known text address.
+		 */
+		if (kernel_text_address(from))
+			addr = (void *)from;
+		else
+			return X86_BR_NONE;
+	}
 
 	/*
 	 * decoder needs to know the ABI especially