[Qemu-devel] [PATCH] check SCSI read/write requests against max LBA

[Rik van Riel <riel@redhat.com>]

The bdrv layer uses a signed offset. Furthermore, block-raw-posix
only seeks when that offset is positive. Passing a negative offset
to block-raw-posix can result in data being written at the current
seek cursor's position.

It may be possible to exploit this to seek to the end of the disk
and extend the virtual disk by writing data to a negative sector
offset.  After a reboot, this could lead to the guest having a
larger disk than it had before.

Close the hole by sanity checking the lba against the size of the
disk.

Signed-off-by: Rik van Riel <riel@redhat.com>

Index: qemu/hw/scsi-disk.c
===================================================================
--- qemu.orig/hw/scsi-disk.c
+++ qemu/hw/scsi-disk.c
@@ -67,6 +67,7 @@ struct SCSIDeviceState
     /* The qemu block layer uses a fixed 512 byte sector size.
        This is the number of 512 byte blocks in a single scsi sector.  */
     int cluster_size;
+    uint64_t max_lba;
     int sense;
     int tcq;
     /* Completion functions may be called from either scsi_{read,write}_data
@@ -738,6 +739,8 @@ static int32_t scsi_send_command(SCSIDev
         /* Returned value is the address of the last sector.  */
         if (nb_sectors) {
             nb_sectors--;
+            /* Remember the new size for read/write sanity checking. */
+            s->max_lba = nb_sectors;
             /* Clip to 2TB, instead of returning capacity modulo 2TB. */
             if (nb_sectors > UINT32_MAX)
                 nb_sectors = UINT32_MAX;
@@ -759,6 +762,8 @@ static int32_t scsi_send_command(SCSIDev
     case 0x28:
     case 0x88:
         DPRINTF("Read (sector %lld, count %d)\n", lba, len);
+        if (lba > s->max_lba)
+            goto illegal_lba;
         r->sector = lba * s->cluster_size;
         r->sector_count = len * s->cluster_size;
         break;
@@ -766,6 +771,8 @@ static int32_t scsi_send_command(SCSIDev
     case 0x2a:
     case 0x8a:
         DPRINTF("Write (sector %lld, count %d)\n", lba, len);
+        if (lba > s->max_lba)
+            goto illegal_lba;
         r->sector = lba * s->cluster_size;
         r->sector_count = len * s->cluster_size;
         is_write = 1;
@@ -839,6 +847,8 @@ static int32_t scsi_send_command(SCSIDev
             /* Returned value is the address of the last sector.  */
             if (nb_sectors) {
                 nb_sectors--;
+                /* Remember the new size for read/write sanity checking. */
+                s->max_lba = nb_sectors;
                 outbuf[0] = (nb_sectors >> 56) & 0xff;
                 outbuf[1] = (nb_sectors >> 48) & 0xff;
                 outbuf[2] = (nb_sectors >> 40) & 0xff;
@@ -877,6 +887,9 @@ static int32_t scsi_send_command(SCSIDev
     fail:
         scsi_command_complete(r, STATUS_CHECK_CONDITION, SENSE_ILLEGAL_REQUEST);
 	return 0;
+    illegal_lba:
+        scsi_command_complete(r, STATUS_CHECK_CONDITION, SENSE_HARDWARE_ERROR);
+        return 0;
     }
     if (r->sector_count == 0 && r->buf_len == 0) {
         scsi_command_complete(r, STATUS_GOOD, SENSE_NO_SENSE);
@@ -902,6 +915,7 @@ SCSIDevice *scsi_disk_init(BlockDriverSt
 {
     SCSIDevice *d;
     SCSIDeviceState *s;
+    uint64_t nb_sectors;
 
     s = (SCSIDeviceState *)qemu_mallocz(sizeof(SCSIDeviceState));
     s->bdrv = bdrv;
@@ -913,6 +927,11 @@ SCSIDevice *scsi_disk_init(BlockDriverSt
     } else {
         s->cluster_size = 1;
     }
+    bdrv_get_geometry(s->bdrv, &nb_sectors);
+    nb_sectors /= s->cluster_size;
+    if (nb_sectors)
+        nb_sectors--;
+    s->max_lba = nb_sectors;
     strncpy(s->drive_serial_str, drive_get_serial(s->bdrv),
             sizeof(s->drive_serial_str));
     if (strlen(s->drive_serial_str) == 0)