Re: [PATCH v2 0/5] hw/scsi/spapr_vscsi: Fix time bomb zero-length array use

[Paolo Bonzini]

On 05/03/20 09:31, Philippe Mathieu-Daudé wrote:
> This series fixes a dangerous zero-length array use.
> Simples patches first to clean the issue in the last patch:
> dissociate the buffer holding DMA requests with pointer to
> SRP Information Unit packets.
> 
> v2: Addressed David Gibson review comments
> 
> Philippe Mathieu-Daudé (5):
>   hw/scsi/viosrp: Add missing 'hw/scsi/srp.h' include
>   hw/scsi/spapr_vscsi: Use SRP_MAX_IU_LEN instead of sizeof flexible
>     array
>   hw/scsi/spapr_vscsi: Simplify a bit
>   hw/scsi/spapr_vscsi: Introduce req_iu() helper
>   hw/scsi/spapr_vscsi: Do not mix SRP IU size with DMA buffer size
> 
>  hw/scsi/viosrp.h      |  3 ++-
>  hw/scsi/spapr_vscsi.c | 59 ++++++++++++++++++++++++-------------------
>  2 files changed, 35 insertions(+), 27 deletions(-)
> 

I think this is clearer than the "reserved" member, and it gets one
important thing right though by decoupling SRP_MAX_IU_LEN from
sizeof(union srp_iu).

However it doesn't fix the actual time bomb, which is that,
vscsi_send_rsp can overrun the buffer size here depending on the length
of sense data:

        memcpy(req->iu.srp.rsp.data, req->sense, req->senselen);

But with this series in place we can fix that with just a couple of
asserts, like this:

/* This #define is why Phil's changes matter! */
#define SRP_MAX_IU_DATA_LEN (SRP_MAX_IU_LEN - sizeof(union srp_iu))

        /* in vscsi_send_rsp */
        int sense_data_len = MIN(req->senselen, SRP_MAX_IU_DATA_LEN);
        req->iu.srp.rsp.sense_data_len = cpu_to_be32(sense_data_len);
        memcpy(req->iu.srp.rsp.data, req->sense, sense_data_len);

        /* in vscsi_send_iu */
        assert(length < SRP_MAX_IU_LEN);

        /* in vscsi_process_tsk_mgmt */

        QEMU_BUILD_BUG_ON(SRP_MAX_IU_DATA_LEN < 4);

(Easier to write code than describe...).

Paolo