[PATCH v3] nvmet-tcp: set and enforce a default MDTS for TCP transport

[Shivam Kumar <kumar.shivam43666@gmail.com>]

Unlike other fabrics transports, the TCP target does not set a default
Maximum Data Transfer Size. With the configfs MDTS entry defaulting to 0
(no limit), a remote attacker can send a CapsuleCmd with an arbitrarily
large SGL length, causing sgl_alloc() in nvmet_tcp_map_data() to attempt
an excessive kernel allocation that triggers the OOM killer.

Set a default MDTS of 9 (2 MiB) for TCP. Enforce the limit server-side
in nvmet_tcp_map_data() by rejecting commands whose SGL length exceeds
the configured MDTS, returning NVME_SC_INVALID_FIELD as required by the
NVMe specification. Admins can still adjust via the configfs mdts
attribute if needed.

Signed-off-by: Shivam Kumar <kumar.shivam43666@gmail.com>
---
 drivers/nvme/target/tcp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index 164a564ba3b4..098cf877c358 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -25,6 +25,7 @@
 #define NVMET_TCP_DEF_INLINE_DATA_SIZE	(4 * PAGE_SIZE)
 #define NVMET_TCP_MAXH2CDATA		0x400000 /* 16M arbitrary limit */
 #define NVMET_TCP_BACKLOG 128
+#define NVMET_TCP_DEF_MDTS		9 /* 2 MiB (2^(12+9)) */
 
 static int param_store_val(const char *str, int *val, int min, int max)
 {
@@ -422,6 +423,13 @@ static int nvmet_tcp_map_data(struct nvmet_tcp_cmd *cmd)
 	if (!len)
 		return 0;
 
+	/* Enforce MDTS: abort commands exceeding the advertised limit */
+	if (cmd->req.port->mdts) {
+		u8 mdts = cmd->req.port->mdts;
+		if (mdts < 20 && len > (1U << (12 + mdts)))
+			return NVME_SC_INVALID_FIELD | NVME_STATUS_DNR;
+	}
+
 	if (sgl->type == ((NVME_SGL_FMT_DATA_DESC << 4) |
 			  NVME_SGL_FMT_OFFSET)) {
 		if (!nvme_is_write(cmd->req.cmd))
@@ -2077,6 +2085,8 @@ static int nvmet_tcp_add_port(struct nvmet_port *nport)
 	INIT_WORK(&port->accept_work, nvmet_tcp_accept_work);
 	if (port->nport->inline_data_size < 0)
 		port->nport->inline_data_size = NVMET_TCP_DEF_INLINE_DATA_SIZE;
+	if (nport->mdts < 0)
+		nport->mdts = NVMET_TCP_DEF_MDTS;
 
 	ret = sock_create(port->addr.ss_family, SOCK_STREAM,
 				IPPROTO_TCP, &port->sock);
-- 
2.34.1