[PATCH v3 05/11] mm/util: Fix possible race condition in kstrdup()

Yafang Shao
2024-06-21T02:31:30Z

In kstrdup(), it is critical to ensure that the dest string is always
NUL-terminated. However, potential race condidtion 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 calculate 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.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
mm/util.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index c9e519e6811f..41c7875572ed 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -60,8 +60,14 @@ 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 null termimator.
+ */
+ buf[len - 1] = '\0';
+ }
return buf;
}
EXPORT_SYMBOL(kstrdup);
--
2.39.1