kcov: update pos before writing pc in trace function

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().

Link: https://lkml.kernel.org/r/20220523053531.1572793-1-liu3101@purdue.edu
Signed-off-by: Congyu Liu <liu3101@purdue.edu>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/kcov.c b/kernel/kcov.c
index b3732b210593086b6402090df6ed3cf1c4a53d1b..e19c84b02452eb2d0c0fa46ac3248535c6a81736 100644 (file)
--- 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);
        }
 }