[PATCH 2/3] perf, x86, lbr: Fix LBR filter

[Peter Zijlstra]

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 either stop using modules as
      all sane people do or optimize it using an RCU backed rb-tree.

Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: eranian@xxxxxxxxxx
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/n/tip-mk8i82ffzax01cnqo829iy1q@xxxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/perf_event_intel_lbr.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -442,8 +442,18 @@ static int branch_type(unsigned long fro
 			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


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