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

[Carlos Bilbao <carlos.bilbao.osdev@gmail.com>]

Hello,

On 4/7/26 02:23, Dmitry Bogdanov wrote:
> 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).


For sure, I'll send a v2 including that.


>
>> +               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);
>

Agreed!


>>                  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
>>

Thanks,

Carlos