Re: [Qemu-devel] [PATCH v2] scsi: pvscsi: check request descriptor SG element count

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/09/2016 22:50, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
> 
> In PVSCSI paravirtual SCSI bus, the request descriptor data
> length is defined to be 64 bit. While building SG list from
> a request descriptor, it gets truncated to 32bit in routine
> 'pvscsi_convert_sglist'. This could lead to an infinite loop
> situation for arbitrarily large 'dataLen' values. Check
> SG list element count to avoid  it.

The commit message is not correct, because you're fixing the bug in two
different ways: by removing the cast and by limiting the number of
iterations in the loop.

---
In PVSCSI paravirtual SCSI bus, pvscsi_convert_sglist can take a very
long time or go into an infinite loop due to two different bugs:

1) the request descriptor data length is defined to be 64 bit. While
building SG list from a request descriptor, it gets truncated to 32bit
in routine 'pvscsi_convert_sglist'. This could lead to an infinite loop
situation large 'dataLen' values when data_length is cast to uint32_t
and chunk_size becomes always zero.  Fix this by removing the incorrect
cast.

2) pvscsi_get_next_sg_elem can be called arbitrarily many times if the
element has a zero length.  Get out of the loop early when this happens,
by introducing an upper limit on the number of SG list elements.
---

Paolo

> Reported-by: Li Qiang <liqiang6-s@360.cn>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  hw/scsi/vmw_pvscsi.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> Update per:
>   -> https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg00594.html
> 
> diff --git a/hw/scsi/vmw_pvscsi.c b/hw/scsi/vmw_pvscsi.c
> index 4245c15..babac5a 100644
> --- a/hw/scsi/vmw_pvscsi.c
> +++ b/hw/scsi/vmw_pvscsi.c
> @@ -40,6 +40,8 @@
>  #define PVSCSI_MAX_DEVS                   (64)
>  #define PVSCSI_MSIX_NUM_VECTORS           (1)
>  
> +#define PVSCSI_MAX_SG_ELEM                2048
> +
>  #define PVSCSI_MAX_CMD_DATA_WORDS \
>      (sizeof(PVSCSICmdDescSetupRings)/sizeof(uint32_t))
>  
> @@ -628,17 +630,16 @@ pvscsi_queue_pending_descriptor(PVSCSIState *s, SCSIDevice **d,
>  static void
>  pvscsi_convert_sglist(PVSCSIRequest *r)
>  {
> -    int chunk_size;
> +    uint32_t chunk_size, elmcnt = 0;
>      uint64_t data_length = r->req.dataLen;
>      PVSCSISGState sg = r->sg;
> -    while (data_length) {
> -        while (!sg.resid) {
> +    while (data_length && elmcnt < PVSCSI_MAX_SG_ELEM) {
> +        while (!sg.resid && elmcnt++ < PVSCSI_MAX_SG_ELEM) {
>              pvscsi_get_next_sg_elem(&sg);
>              trace_pvscsi_convert_sglist(r->req.context, r->sg.dataAddr,
>                                          r->sg.resid);
>          }
> -        assert(data_length > 0);
> -        chunk_size = MIN((unsigned) data_length, sg.resid);
> +        chunk_size = MIN(data_length, sg.resid);
>          if (chunk_size) {
>              qemu_sglist_add(&r->sgl, sg.dataAddr, chunk_size);
>          }
>