In kstrdup(), it is critical to ensure that the dest string is always
NUL-terminated. However, potential race condition can occur between a
writer and a reader.
Consider the following scenario involving task->comm:
reader writer
len = strlen(s) + 1;
strlcpy(tsk->comm, buf, sizeof(tsk->comm));
memcpy(buf, s, len);
In this case, there is a race condition between the reader and the
writer. The reader calculates the length of the string `s` based on the
old value of task->comm. However, during the memcpy(), the string `s`
might be updated by the writer to a new value of task->comm.
If the new task->comm is larger than the old one, the `buf` might not be
NUL-terminated. This can lead to undefined behavior and potential
security vulnerabilities.
Let's fix it by explicitly adding a NUL terminator after the memcpy. It
is worth noting that memcpy() is not atomic, so the new string can be
shorter when memcpy() already copied past the new NUL.
Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Alejandro Colomar <alx@xxxxxxxxxx>
Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
---
mm/util.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/mm/util.c b/mm/util.c
index 4f1275023eb7..858a9a2f57e7 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -62,8 +62,15 @@ char *kstrdup(const char *s, gfp_t gfp)
len = strlen(s) + 1;
buf = kmalloc_track_caller(len, gfp);
- if (buf)
+ if (buf) {
memcpy(buf, s, len);
+ /*
+ * During memcpy(), the string might be updated to a new value,
+ * which could be longer than the string when strlen() is
+ * called. Therefore, we need to add a NUL terminator.
+ */
+ buf[len - 1] = '\0';
+ }
return buf;
}
EXPORT_SYMBOL(kstrdup);
--
2.43.5
