[Qemu-devel] [PATCH 5/5] scsi-disk: lazily allocate bounce buffer

[Paolo Bonzini <pbonzini@redhat.com>]

It will not be needed for reads and writes if the HBA provides a sglist.
In addition, this lets scsi-disk refuse commands with an excessive
allocation length, as well as limit memory on usual well-behaved guests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/scsi-disk.c |   44 +++++++++++++++++++++++++++++++++-----------
 1 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/hw/scsi-disk.c b/hw/scsi-disk.c
index d0ac31e..4a01e0f 100644
--- a/hw/scsi-disk.c
+++ b/hw/scsi-disk.c
@@ -54,6 +54,7 @@ typedef struct SCSIDiskReq {
     /* Both sector and sector_count are in terms of qemu 512 byte blocks.  */
     uint64_t sector;
     uint32_t sector_count;
+    uint32_t buflen;
     struct iovec iov;
     QEMUIOVector qiov;
     uint32_t status;
@@ -75,13 +76,15 @@ struct SCSIDiskState
 };
 
 static int scsi_handle_rw_error(SCSIDiskReq *r, int error, int type);
-static int scsi_disk_emulate_command(SCSIDiskReq *r, uint8_t *outbuf);
+static int scsi_disk_emulate_command(SCSIDiskReq *r);
 
 static void scsi_free_request(SCSIRequest *req)
 {
     SCSIDiskReq *r = DO_UPCAST(SCSIDiskReq, req, req);
 
-    qemu_vfree(r->iov.iov_base);
+    if (r->iov.iov_base) {
+        qemu_vfree(r->iov.iov_base);
+    }
 }
 
 /* Helper function for command completion with sense.  */
@@ -107,7 +110,13 @@ static void scsi_cancel_io(SCSIRequest *req)
 
 static uint32_t scsi_init_iovec(SCSIDiskReq *r)
 {
-    r->iov.iov_len = MIN(r->sector_count * 512, SCSI_DMA_BUF_SIZE);
+    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
+
+    if (!r->iov.iov_base) {
+        r->buflen = SCSI_DMA_BUF_SIZE;
+        r->iov.iov_base = qemu_blockalign(s->bs, r->buflen);
+    }
+    r->iov.iov_len = MIN(r->sector_count * 512, r->buflen);
     qemu_iovec_init_external(&r->qiov, &r->iov, 1);
     return r->qiov.size / 512;
 }
@@ -314,7 +323,7 @@ static void scsi_dma_restart_bh(void *opaque)
                 scsi_write_data(&r->req);
                 break;
             case SCSI_REQ_STATUS_RETRY_FLUSH:
-                ret = scsi_disk_emulate_command(r, r->iov.iov_base);
+                ret = scsi_disk_emulate_command(r);
                 if (ret == 0) {
                     scsi_req_complete(&r->req, GOOD);
                 }
@@ -808,13 +817,31 @@ static int scsi_disk_emulate_read_toc(SCSIRequest *req, uint8_t *outbuf)
     return toclen;
 }
 
-static int scsi_disk_emulate_command(SCSIDiskReq *r, uint8_t *outbuf)
+static int scsi_disk_emulate_command(SCSIDiskReq *r)
 {
     SCSIRequest *req = &r->req;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, req->dev);
     uint64_t nb_sectors;
+    uint8_t *outbuf;
     int buflen = 0;
 
+    if (!r->iov.iov_base) {
+        /*
+         * FIXME: we shouldn't return anything bigger than 4k, but the code
+         * requires the buffer to be as big as req->cmd.xfer in several
+         * places.  So, do not allow CDBs with a very large ALLOCATION
+         * LENGTH.  The real fix would be to modify scsi_read_data and
+         * dma_buf_read, so that they return data beyond the buflen
+         * as all zeros.
+         */
+        if (req->cmd.xfer > 65536) {
+            goto illegal_request;
+        }
+        r->buflen = MAX(4096, req->cmd.xfer);
+        r->iov.iov_base = qemu_blockalign(s->bs, r->buflen);
+    }
+
+    outbuf = r->iov.iov_base;
     switch (req->cmd.buf[0]) {
     case TEST_UNIT_READY:
         if (!bdrv_is_inserted(s->bs))
@@ -965,11 +992,9 @@ static int32_t scsi_send_command(SCSIRequest *req, uint8_t *buf)
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, req->dev);
     int32_t len;
     uint8_t command;
-    uint8_t *outbuf;
     int rc;
 
     command = buf[0];
-    outbuf = (uint8_t *)r->iov.iov_base;
     DPRINTF("Command: lun=%d tag=0x%x data=0x%02x", req->lun, req->tag, buf[0]);
 
 #ifdef DEBUG_SCSI
@@ -998,7 +1023,7 @@ static int32_t scsi_send_command(SCSIRequest *req, uint8_t *buf)
     case GET_CONFIGURATION:
     case SERVICE_ACTION_IN_16:
     case VERIFY_10:
-        rc = scsi_disk_emulate_command(r, outbuf);
+        rc = scsi_disk_emulate_command(r);
         if (rc < 0) {
             return 0;
         }
@@ -1228,11 +1253,8 @@ static SCSIRequest *scsi_new_request(SCSIDevice *d, uint32_t tag,
 {
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, d);
     SCSIRequest *req;
-    SCSIDiskReq *r;
 
     req = scsi_req_alloc(&scsi_disk_reqops, &s->qdev, tag, lun, hba_private);
-    r = DO_UPCAST(SCSIDiskReq, req, req);
-    r->iov.iov_base = qemu_blockalign(s->bs, SCSI_DMA_BUF_SIZE);
     return req;
 }
 
-- 
1.7.6