[PATCH -next v2] cifsd: check return value of ksmbd_vfs_getcasexattr() correctly

[Yang Yingliang]

If ksmbd_vfs_getcasexattr() returns -ENOMEM, stream_buf is NULL,
it will cause null-ptr-deref when using it to copy memory. So we
need check the return value of ksmbd_vfs_getcasexattr() by comparing
with 0.

Fixes: f44158485826 ("cifsd: add file operations")
Signed-off-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
---
v2:
  Handle the case ksmbd_vfs_getcasexattr() returns 0.
---
 fs/cifsd/vfs.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/cifsd/vfs.c b/fs/cifsd/vfs.c
index 97d5584ec870..2a9cc0bc7726 100644
--- a/fs/cifsd/vfs.c
+++ b/fs/cifsd/vfs.c
@@ -274,7 +274,6 @@ static int ksmbd_vfs_stream_read(struct ksmbd_file *fp, char *buf, loff_t *pos,
 {
 	ssize_t v_len;
 	char *stream_buf = NULL;
-	int err;
 
 	ksmbd_debug(VFS, "read stream data pos : %llu, count : %zd\n",
 		    *pos, count);
@@ -283,10 +282,9 @@ static int ksmbd_vfs_stream_read(struct ksmbd_file *fp, char *buf, loff_t *pos,
 				       fp->stream.name,
 				       fp->stream.size,
 				       &stream_buf);
-	if (v_len == -ENOENT) {
+	if ((int)v_len <= 0) {
 		ksmbd_err("not found stream in xattr : %zd\n", v_len);
-		err = -ENOENT;
-		return err;
+		return v_len == 0 ? -ENOENT : (int)v_len;
 	}
 
 	memcpy(buf, &stream_buf[*pos], count);
@@ -415,9 +413,9 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 				       fp->stream.name,
 				       fp->stream.size,
 				       &stream_buf);
-	if (v_len == -ENOENT) {
+	if ((int)v_len <= 0) {
 		ksmbd_err("not found stream in xattr : %zd\n", v_len);
-		err = -ENOENT;
+		err = v_len == 0 ? -ENOENT : (int)v_len;
 		goto out;
 	}
 
-- 
2.25.1