[PATCH 2/3] perf, x86: Don't leak kernel data via callchains

[Arun Sharma <asharma@fb.com>]

The NMI could fire when the kernel has done set_fs(KERNEL_DS).
Since we're interested in user space stack pointers only,
check against USER_DS explicitly.

Signed-off-by: Arun Sharma <asharma@fb.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/kernel/cpu/perf_event.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 9f98636..ccd8c07 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1803,6 +1803,7 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 {
 	struct stack_frame frame;
 	const void __user *fp;
+	mm_segment_t oldfs = get_fs();
 
 	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
 		/* TODO: We don't support guest os callchain now */
@@ -1816,8 +1817,9 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 	if (!current->mm)
 		return;
 
+	set_fs(USER_DS);
 	if (perf_callchain_user32(regs, entry))
-		return;
+		goto out;
 
 	while (entry->nr < PERF_MAX_STACK_DEPTH) {
 		unsigned long bytes;
@@ -1831,6 +1833,10 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 		perf_callchain_store(entry, frame.return_address);
 		fp = frame.next_frame;
 	}
+
+out:
+	set_fs(oldfs);
+	return;
 }
 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
-- 
1.7.8.4