[PATCH v1 2/2] perf/x86/intel/bts: Disallow use by unprivileged users on paranoid systems

From: Alexander Shishkin
Date: Mon Aug 31 2015 - 10:11:55 EST

BTS leaks kernel addresses even in userspace-only mode due to imprecise IP
sampling, so sometimes syscall entry points or page fault handler addresses
end up in a userspace trace.

Now, intel_bts driver exports trace data zero-copy, it does not scan through
it to filter out the kernel addresses and it's would be a O(n) job.

To work around this situation, this patch forbids the use of intel_bts
driver by unprivileged users on systems with the paranoid setting above the
(kernel's) default "1", which still allows kernel profiling. In other words,
using intel_bts driver implies kernel tracing, regardless of the
"exclude_kernel" attribute setting.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
arch/x86/kernel/cpu/perf_event_intel_bts.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_bts.c b/arch/x86/kernel/cpu/perf_event_intel_bts.c
index 80df16e020..014b0c34cd 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_bts.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_bts.c
@@ -495,6 +495,19 @@ static int bts_event_init(struct perf_event *event)
if (x86_add_exclusive(x86_lbr_exclusive_bts))
return -EBUSY;

+ /*
+ * BTS leaks kernel addresses even when CPL0 tracing is
+ * disabled, so disallow intel_bts driver for unprivileged
+ * users on paranoid systems since it provides trace data
+ * to the user in a zero-copy fashion.
+ *
+ * Note that the default paranoia setting permits unprivileged
+ * users to profile the kernel.
+ */
+ if (event->attr.exclude_kernel && perf_paranoid_kernel() &&
+ !capable(CAP_SYS_ADMIN))
+ return -EACCES;
ret = x86_reserve_hardware();
if (ret) {

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