[PATCH v2] kcov: update pos before writing pc in trace function

[Congyu Liu]

In __sanitizer_cov_trace_pc(), previously we write pc before updating pos.
However, some early interrupt code could bypass check_kcov_mode()
check and invoke __sanitizer_cov_trace_pc(). If such interrupt is raised
between writing pc and updating pos, the pc could be overitten by the
recursive __sanitizer_cov_trace_pc().

As suggested by Dmitry, we cold update pos before writing pc to avoid
such interleaving.

Apply the same change to write_comp_data().

Signed-off-by: Congyu Liu <liu3101@xxxxxxxxxx>
---
PATCH v2:
* Update pos before writing pc as suggested by Dmitry.

PATCH v1:
https://lore.kernel.org/lkml/20220517210532.1506591-1-liu3101@xxxxxxxxxx/
---
 kernel/kcov.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index b3732b210593..e19c84b02452 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -204,8 +204,16 @@ void notrace __sanitizer_cov_trace_pc(void)
 	/* The first 64-bit word is the number of subsequent PCs. */
 	pos = READ_ONCE(area[0]) + 1;
 	if (likely(pos < t->kcov_size)) {
-		area[pos] = ip;
+		/* Previously we write pc before updating pos. However, some
+		 * early interrupt code could bypass check_kcov_mode() check
+		 * and invoke __sanitizer_cov_trace_pc(). If such interrupt is
+		 * raised between writing pc and updating pos, the pc could be
+		 * overitten by the recursive __sanitizer_cov_trace_pc().
+		 * Update pos before writing pc to avoid such interleaving.
+		 */
 		WRITE_ONCE(area[0], pos);
+		barrier();
+		area[pos] = ip;
 	}
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
@@ -236,11 +244,13 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 	start_index = 1 + count * KCOV_WORDS_PER_CMP;
 	end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
 	if (likely(end_pos <= max_pos)) {
+		/* See comment in __sanitizer_cov_trace_pc(). */
+		WRITE_ONCE(area[0], count + 1);
+		barrier();
 		area[start_index] = type;
 		area[start_index + 1] = arg1;
 		area[start_index + 2] = arg2;
 		area[start_index + 3] = ip;
-		WRITE_ONCE(area[0], count + 1);
 	}
 }
 
-- 
2.34.1