Re: [PATCH v4] libceph, ceph: reject oversized mon and MDS data segments

[Viacheslav Dubeyko]

On Mon, 2026-05-04 at 13:16 +0400, Dhiraj Mishra wrote:
> Monitor and MDS messages can be allocated with only front-buffer storage
> and no data items.  The messenger receive path copies the wire header
> into the selected ceph_msg and later uses hdr.data_len to decide whether
> to initialize a data cursor.
> 
> If a malicious or compromised peer advertises a non-zero data segment for
> one of these front-only messages, the receive path can call
> ceph_msg_data_cursor_init() with length greater than msg->data_length and
> hit its BUG_ON() checks, crashing the client kernel.
> 
> I verified the monitor issue against v7.1-rc1-123-ge75a43c7cec4.  The
> msgr2 trigger path is present since commit cd1a677cad99 ("libceph,
> ceph: implement msgr2.1 protocol (crc and secure modes)"), which is
> contained in v5.11-rc1 and later.  The allocator patterns are older, but
> I have not tested older msgr1-only kernels.
> 
> A concrete monitor trigger is a monitor connection over msgr2 after
> CEPH_CON_S_OPEN where a FRAME_TAG_MESSAGE contains a monitor reply type
> handled by mon_alloc_msg(), a valid front_len for that message type and
> data_len = 1.  CEPH_MSG_MON_MAP is one such example: the message is
> allocated with ceph_msg_new(), leaving msg->data_length and
> msg->num_data_items as zero.
> 
> The MDS allocation path has the same issue: mds_alloc_msg() allocates the
> incoming message with ceph_msg_new(type, front_len, GFP_NOFS, false),
> leaving msg->data_length as zero for a front-only message selected from
> an attacker-controlled wire header.
> 
> Reject monitor and MDS messages whose wire data segment is larger than
> the data backing allocated for the selected ceph_msg, mirroring the
> existing OSD reply hardening.
> 
> Fixes: 53ded495c6ac ("libceph: define mds_alloc_msg() method")
> Fixes: cd1a677cad99 ("libceph, ceph: implement msgr2.1 protocol (crc and secure modes)")
> Signed-off-by: Dhiraj Mishra <mishra.dhiraj95@xxxxxxxxx>
> ---
> v4:
>   - Add the same oversized data_len guard to the MDS allocation path.
> 
> v3:
>   - Remove the impossible !req->reply check for generic requests.
>   - Use pr_warn_ratelimited() for malicious-monitor log spam resistance.
>   - Avoid adding __func__ to the new mon_client warnings.
>   - Add a blank line between the front_len and data_len checks.
> 
> v2:
>   - Resend as an inline plain-text patch.
>   - Add full email address to the From and Signed-off-by identities.
>   - Add ceph-devel and LKML to the recipient list when sending.
> 
>  fs/ceph/mds_client.c  | 13 +++++++++++++
>  net/ceph/mon_client.c | 17 +++++++++++++++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index ed17e0023705..3b92d37983d1 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -6543,9 +6543,12 @@ static int mds_handle_auth_bad_method(struct ceph_connection *con,
>  static struct ceph_msg *mds_alloc_msg(struct ceph_connection *con,
>  				struct ceph_msg_header *hdr, int *skip)
>  {
> +	struct ceph_mds_session *s = con->private;
> +	struct ceph_client *cl = s->s_mdsc->fsc->client;
>  	struct ceph_msg *msg;
>  	int type = (int) le16_to_cpu(hdr->type);
>  	int front_len = (int) le32_to_cpu(hdr->front_len);
> +	u32 data_len = le32_to_cpu(hdr->data_len);
>  
>  	if (con->in_msg)
>  		return con->in_msg;
> @@ -6558,6 +6561,16 @@ static struct ceph_msg *mds_alloc_msg(struct ceph_connection *con,
>  		return NULL;
>  	}
>  
> +	if (data_len > msg->data_length) {
> +		pr_warn_ratelimited_client(cl,
> +					   "mds%d message data %u > prealloc %zu, skipping\n",
> +					   s->s_mds, data_len,
> +					   msg->data_length);
> +		ceph_msg_put(msg);
> +		*skip = 1;
> +		return NULL;
> +	}
> +
>  	return msg;
>  }
>  
> diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
> index d2cdc8ee3155..9f1c7ca42f36 100644
> --- a/net/ceph/mon_client.c
> +++ b/net/ceph/mon_client.c
> @@ -712,6 +712,7 @@ static struct ceph_msg *get_generic_reply(struct ceph_connection *con,
>  	struct ceph_mon_client *monc = con->private;
>  	struct ceph_mon_generic_request *req;
>  	u64 tid = le64_to_cpu(hdr->tid);
> +	u32 data_len = le32_to_cpu(hdr->data_len);
>  	struct ceph_msg *m;
>  
>  	mutex_lock(&monc->mutex);
> @@ -720,6 +721,11 @@ static struct ceph_msg *get_generic_reply(struct ceph_connection *con,
>  		dout("get_generic_reply %lld dne\n", tid);
>  		*skip = 1;
>  		m = NULL;
> +	} else if (data_len > req->reply->data_length) {
> +		pr_warn_ratelimited("mon generic reply tid %llu data %u > preallocated %zu, skipping\n",
> +				    tid, data_len, req->reply->data_length);
> +		*skip = 1;
> +		m = NULL;
>  	} else {
>  		dout("get_generic_reply %lld got %p\n", tid, req->reply);
>  		*skip = 0;
> @@ -1499,6 +1505,7 @@ static struct ceph_msg *mon_alloc_msg(struct ceph_connection *con,
>  	struct ceph_mon_client *monc = con->private;
>  	int type = le16_to_cpu(hdr->type);
>  	int front_len = le32_to_cpu(hdr->front_len);
> +	u32 data_len = le32_to_cpu(hdr->data_len);
>  	struct ceph_msg *m = NULL;
>  
>  	*skip = 0;
> @@ -1545,5 +1552,15 @@ static struct ceph_msg *mon_alloc_msg(struct ceph_connection *con,
>  		m = ceph_msg_new(type, front_len, GFP_NOFS, false);
>  	}
>  
> +	if (m && data_len > m->data_length) {
> +		pr_warn_ratelimited("mon message data %u > prealloc %zu (%u#%llu), skipping\n",
> +				    data_len, m->data_length,
> +				    (unsigned int)con->peer_name.type,
> +				    le64_to_cpu(con->peer_name.num));
> +		ceph_msg_put(m);
> +		m = NULL;
> +		*skip = 1;
> +	}
> +

I still don't see the answer on my question. We have:

	case CEPH_MSG_STATFS_REPLY:
	case CEPH_MSG_MON_COMMAND_ACK:
		return get_generic_reply(con, hdr, skip);

I had impression that this code was related to get_generic_reply() case. But it
looks like that I am not correct here.

If it is not related to this, then we need to rework this code to integrate
suggested new functionality into the whole logic:

	if (!m) {
		pr_info("alloc_msg unknown type %d\n", type);
		*skip = 1;
	} else if (front_len > m->front_alloc_len) {
		pr_warn("mon_alloc_msg front %d > prealloc %d (%u#%llu)\n",
			front_len, m->front_alloc_len,
			(unsigned int)con->peer_name.type,
			le64_to_cpu(con->peer_name.num));
		ceph_msg_put(m);
		m = ceph_msg_new(type, front_len, GFP_NOFS, false);
	}

Thanks,
Slava.


>  	return m;
>  }