[Qemu-devel] [PATCH scsi] scsi-generic: fix sign extension of READ CAPACITY(10) data

[Paolo Bonzini <pbonzini@redhat.com>]

Issuing the READ CAPACITY(10) command in the guest will cause QEMU
to update its knowledge of the maximum accessible LBA in the disk.
The recorded maximum LBA will be wrong if the disk is bigger than
1TB, because ldl_be_p returns a signed int.

When this is fixed, a latent bug will be unmasked.  If the READ
CAPACITY(10) command reported an overflow (0xFFFFFFFF), we must
not overwrite the previously-known maximum accessible LBA, or the guest
will fail to access the disk above the first 2TB.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/scsi/scsi-generic.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
index 2a9a561..19bd36c 100644
--- a/hw/scsi/scsi-generic.c
+++ b/hw/scsi/scsi-generic.c
@@ -198,9 +198,10 @@ static void scsi_read_complete(void * opaque, int ret)
         scsi_command_complete(r, 0);
     } else {
         /* Snoop READ CAPACITY output to set the blocksize.  */
-        if (r->req.cmd.buf[0] == READ_CAPACITY_10) {
+        if (r->req.cmd.buf[0] == READ_CAPACITY_10 &&
+            (ldl_be_p(&r->buf[0]) != 0xffffffffU || s->max_lba == 0)) {
             s->blocksize = ldl_be_p(&r->buf[4]);
-            s->max_lba = ldl_be_p(&r->buf[0]);
+            s->max_lba = ldl_be_p(&r->buf[0]) & 0xffffffffULL;
         } else if (r->req.cmd.buf[0] == SERVICE_ACTION_IN_16 &&
                    (r->req.cmd.buf[1] & 31) == SAI_READ_CAPACITY_16) {
             s->blocksize = ldl_be_p(&r->buf[8]);
-- 
1.8.1.4