Re: [PATCH] scsi: target: iscsi: reject zero-length Extended CDB AHS

Dmitry Bogdanov
2026-04-07T09:32:35Z

On Fri, Apr 03, 2026 at 06:44:29PM -0700, carlos.bilbao@kernel.org wrote:
> From: Carlos Bilbao <carlos.bilbao@kernel.org>
>
> If ecdb_ahdr->ahslength is zero, two bugs follow:
>
> kmalloc(be16_to_cpu(ecdb_ahdr->ahslength) + 15, ...)
>
> allocates 15 bytes, but the immediately following memcpy writes
> ISCSI_CDB_SIZE (16) bytes into it, a one-byte heap overflow. Also:
>
> memcpy(cdb + ISCSI_CDB_SIZE, ecdb_ahdr->ecdb,
> be16_to_cpu(ecdb_ahdr->ahslength) - 1);
>
> (u16)0 - 1 promotes to (int)-1 which converts to SIZE_MAX as size_t,
> causing a massive out-of-bounds write.
>
> Reject ahslength == 0 with ISCSI_REASON_PROTOCOL_ERROR before the kmalloc.
>
> Fixes: 8f1f7d297bce ("scsi: target: iscsi: Add support for extended CDB AHS")
> Signed-off-by: Carlos Bilbao (Lambda) <carlos.bilbao@kernel.org>
> ---
> drivers/target/iscsi/iscsi_target.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
> index e80449f6ce15..8db24d35c762 100644
> --- a/drivers/target/iscsi/iscsi_target.c
> +++ b/drivers/target/iscsi/iscsi_target.c
> @@ -1100,6 +1100,8 @@ int iscsit_setup_scsi_cmd(struct iscsit_conn *conn, struct iscsit_cmd *cmd,
> cdb = hdr->cdb;
>
> if (hdr->hlength) {
> + u16 ahslength;
> +
> ecdb_ahdr = (struct iscsi_ecdb_ahdr *) (hdr + 1);
> if (ecdb_ahdr->ahstype != ISCSI_AHSTYPE_CDB) {
> pr_err("Additional Header Segment type %d not supported!\n",
> @@ -1108,14 +1110,19 @@ int iscsit_setup_scsi_cmd(struct iscsit_conn *conn, struct iscsit_cmd *cmd,
> ISCSI_REASON_CMD_NOT_SUPPORTED, buf);
> }
>
> - cdb = kmalloc(be16_to_cpu(ecdb_ahdr->ahslength) + 15,
> - GFP_KERNEL);
> + ahslength = be16_to_cpu(ecdb_ahdr->ahslength);
> + if (!ahslength) {
> + pr_err("Extended CDB AHS with zero length, protocol error.\n");
> + return iscsit_add_reject_cmd(cmd,
> + ISCSI_REASON_PROTOCOL_ERROR, buf);
> + }
> +

For a complete solution please add a check that AHS fits in the buffer.
ahslength must be less or equal than ((hdr->hlength * 4) - 3).

> + cdb = kmalloc(ahslength + 15, GFP_KERNEL);

It took some time to recall what did 15 mean. May be make it clear for
everyone too?

u16 cdb_length = ahslength - 1 + ISCSI_CDB_SIZE;
cdb = kmalloc(cdb_length, GFP_KERNEL);
memcpy(cdb, hdr->cdb, ISCSI_CDB_SIZE);
memcpy(cdb + ISCSI_CDB_SIZE, ecdb_ahdr->ecdb, cdb_length - ISCSI_CDB_SIZE);

> if (cdb == NULL)
> return iscsit_add_reject_cmd(cmd,
> ISCSI_REASON_BOOKMARK_NO_RESOURCES, buf);
> memcpy(cdb, hdr->cdb, ISCSI_CDB_SIZE);
> - memcpy(cdb + ISCSI_CDB_SIZE, ecdb_ahdr->ecdb,
> - be16_to_cpu(ecdb_ahdr->ahslength) - 1);
> + memcpy(cdb + ISCSI_CDB_SIZE, ecdb_ahdr->ecdb, ahslength - 1);
> }
>
> data_direction = (hdr->flags & ISCSI_FLAG_CMD_WRITE) ? DMA_TO_DEVICE :
> --
> 2.43.0
>