Snapshot longname reconstruction formats "_<name>_<inode>" via snprintf()
and memcpy()s the result into oname->name.  Checking the length against
NAME_MAX only protects the LOOKUPNAME/export callers, whose buffer is
allocated with fscrypt_fname_alloc_buffer(NAME_MAX).  The readdir path
points oname->name straight at the altname in the MDS reply, whose
capacity is altname_len; the reconstructed name can exceed that while
still fitting under NAME_MAX, overflowing the in-place destination.

Save the actual oname->len before the decryption overwrites it, and fail
with -ENAMETOOLONG when the reconstructed name would not fit.

Fixes: dd66df0053ef ("ceph: add support for encrypted snapshot names")
Cc: stable@vger.kernel.org
Suggested-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
---
Changes in v2:
- Save the actual destination capacity (oname->len) before the
  decryption overwrites it, instead of checking only against NAME_MAX.
  The readdir path points oname->name at altname in the MDS reply,
  whose capacity is altname_len; NAME_MAX alone still overflows it.
  (Suggested by Alex Markuze)

 fs/ceph/crypto.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/ceph/crypto.c b/fs/ceph/crypto.c
index bc0a097a4cea..845b69ba890c 100644
--- a/fs/ceph/crypto.c
+++ b/fs/ceph/crypto.c
@@ -313,6 +313,7 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 	struct fscrypt_str iname;
 	char *name = fname->name;
 	int name_len = fname->name_len;
+	int dst_len;
 	int ret;
 
 	if (WARN_ON_ONCE(tname && is_vmalloc_addr(tname)))
@@ -387,6 +388,9 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 		iname.len = fname->ctext_len;
 	}
 
+	/* oname->len is overwritten by the decryption below */
+	dst_len = oname->len;
+
 	_oname.name = unlikely(is_vmalloc_addr(oname->name)) ? tname : oname->name;
 	_oname.len = oname->len;
 
@@ -403,6 +407,10 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
 
 		name_len = snprintf(tmp_buf, sizeof(tmp_buf), "_%.*s_%llu",
 				    oname->len, oname->name, dir->i_ino);
+		if (name_len > dst_len) {
+			ret = -ENAMETOOLONG;
+			goto out;
+		}
 		memcpy(oname->name, tmp_buf, name_len);
 		oname->len = name_len;
 	}
-- 
2.25.1