[PATCH] scsi: target: iscsi: validate ECDB AHS length

[PATCH] scsi: target: iscsi: validate ECDB AHS length

[Jeremy Erazo (Devel Group)]

iscsit_setup_scsi_cmd() processes the Extended-CDB Additional Header
Segment (AHS) of a SCSI Command PDU without bounding AHSLength,
despite the long-standing "FIXME; Add checks for AdditionalHeaderSegment"
comment a few lines above in the same function.

A SCSI Command PDU sent after iSCSI Login with hlength=1,
ahstype=ISCSI_AHSTYPE_CDB and ahslength=0 reaches:

    cdb = kmalloc(0 + 15, GFP_KERNEL);             /* 15-byte alloc  */
    memcpy(cdb, hdr->cdb, ISCSI_CDB_SIZE);         /* 16 -> 15       */
    memcpy(cdb + ISCSI_CDB_SIZE, ecdb_ahdr->ecdb,
           be16_to_cpu(ecdb_ahdr->ahslength) - 1); /* (size_t)-1     */

On CONFIG_FORTIFY_SOURCE=y kernels the first memcpy is rejected by
__fortify_panic() because the declared destination size is 15:

    memcpy: detected buffer overflow: 16 byte write of buffer size 15
    kernel BUG at lib/string_helpers.c:1044!
    Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
    RIP: 0010:__fortify_panic+0xd/0xf
    Call Trace:
     iscsit_setup_scsi_cmd.cold+0x8c/0x224
     iscsit_get_rx_pdu+0x9ec/0x1740
     iscsi_target_rx_thread+0xf7/0x1f0
     kthread+0x1b4/0x200
    Kernel panic - not syncing: Fatal exception

On kernels without CONFIG_FORTIFY_SOURCE the first memcpy fits in the
kmalloc-16 slab object and execution reaches the second memcpy whose
size argument has wrapped to (size_t)-1.

Reproduced on Linux 7.0 with a malformed Command PDU sent after a
completed iSCSI Login.  The trigger is reachable post-Login by any
initiator that successfully logged in (anonymous on demo-mode targets,
authenticated on CHAP-protected targets).  No claim of RCE, LPE or
controlled write is made.

Validate, before any dereference and any allocation:

  - the AHS area received from the socket holds at least the 4-byte
    iscsi_ecdb_ahdr header,
  - AHSLength is at least 1 (RFC 7143 §10.2.2.3 minimum for the ECDB
    AHS, which carries one reserved byte),
  - the declared AHSLength does not exceed the AHS bytes that were
    actually received.

Reported-by: Jeremy Erazo (trexnegr0) <mendozayt13@gmail.com>
Signed-off-by: Jeremy Erazo (Devel Group) <mendozayt13@gmail.com>
Cc: stable@vger.kernel.org
---
 drivers/target/iscsi/iscsi_target.c | 33 +++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index e80449f6c..de291eb6f 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -1100,6 +1100,16 @@ int iscsit_setup_scsi_cmd(struct iscsit_conn *conn, struct iscsit_cmd *cmd,
 	cdb = hdr->cdb;
 
 	if (hdr->hlength) {
+		u16 ahslen;
+		unsigned int ahs_area_bytes = hdr->hlength * 4;
+
+		/* The AHS area must hold at least the iscsi_ecdb_ahdr
+		 * header before any of its fields may be dereferenced.
+		 */
+		if (ahs_area_bytes < sizeof(struct iscsi_ecdb_ahdr))
+			return iscsit_add_reject_cmd(cmd,
+				ISCSI_REASON_PROTOCOL_ERROR, buf);
+
 		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 +1118,29 @@ 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);
+		/* Per RFC 7143 §10.2.2.3 AHSLength counts the bytes of
+		 * the AHS that follow the AHSType/AHSLength fields; for
+		 * the ECDB AHS it includes one reserved byte, so the
+		 * smallest legal value is 1.  Rejecting 0 prevents the
+		 * "ahslen - 1" memcpy size below from underflowing to
+		 * (size_t)-1, and ensures the kmalloc(ahslen + 15) below
+		 * is at least ISCSI_CDB_SIZE (16) so the first memcpy
+		 * does not overflow.  Also reject any AHSLength larger
+		 * than the AHS bytes that actually reached us.
+		 */
+		ahslen = be16_to_cpu(ecdb_ahdr->ahslength);
+		if (ahslen < 1 ||
+		    ahslen - 1 > ahs_area_bytes -
+				 offsetof(struct iscsi_ecdb_ahdr, ecdb))
+			return iscsit_add_reject_cmd(cmd,
+				ISCSI_REASON_PROTOCOL_ERROR, buf);
+
+		cdb = kmalloc(ahslen + 15, GFP_KERNEL);
 		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, ahslen - 1);
 	}
 
 	data_direction = (hdr->flags & ISCSI_FLAG_CMD_WRITE) ? DMA_TO_DEVICE :

base-commit: a293ec25d59dd96309058c70df5a4dd0f889a1e4
-- 
2.53.0