[PATCH 2/2] fs/zfs/zfs.c: zfs_mount() - avoid pointer downcasting

[Jagannathan Raman]

Coverity reports that while loopis in the following functions uses
tainted data as boundary:
  zfs_mount() -> check_mos_features() -> dnode_get() -> zfs_log2()
  zfs_mount() -> grub_memmove()

The defect type is "Untrusted loop bound" caused as a result of
"tainted_data_downcast". Coverity does not like the pointer downcast
here and we need to address it.

We believe Coverity flags pointer downcast for the following two
reasons:
1. External data: The pointer downcast could indicate that the source is
  external data, which we need to further sanitize - such as verifying its
  limits. In this case, the data is read from an external source, which is
  a disk. But, zio_read(), which reads the data from the disk, sanitizes it
  using a checksum. checksum is the best facility that ZFS offers to verify
  external data, and we don't believe a better way exists. Therefore, no
  further action is possible for this.

2. Corruption due to alignment: downcasting a pointer from a strict type
  to less strict type could result in data corruption. For example, the
  following cast would corrupt because uint32_t is 4-byte aligned, and
  won't be able to point to 0x1003 which is not 4-byte aligned.
    uint8_t *ptr = 0x1003;
    uint32_t *word = ptr; (incorrect, alignment issues)

  This patch converts the "osp" pointer in zfs_mount() from a "void" type
  to "objset_phys_t" type to address this issue.

We are not sure if there are any other reasons why Coverity flags the
downcast. However, the fix for alignment issue masks/suppresses any
other issues from showing up.

Fixes: CID 314023

Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
---
 grub-core/fs/zfs/zfs.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/grub-core/fs/zfs/zfs.c b/grub-core/fs/zfs/zfs.c
index d9c79663b..ffa0e5863 100644
--- a/grub-core/fs/zfs/zfs.c
+++ b/grub-core/fs/zfs/zfs.c
@@ -3643,7 +3643,7 @@ zfs_mount (grub_device_t dev)
 {
   struct grub_zfs_data *data = 0;
   grub_err_t err;
-  void *osp = 0;
+  objset_phys_t *osp = 0;
   grub_size_t ospsize;
   grub_zfs_endian_t ub_endian = GRUB_ZFS_UNKNOWN_ENDIAN;
   uberblock_t *ub;
@@ -3681,7 +3681,7 @@ zfs_mount (grub_device_t dev)
               ? GRUB_ZFS_LITTLE_ENDIAN : GRUB_ZFS_BIG_ENDIAN);
 
   err = zio_read (&ub->ub_rootbp, ub_endian,
-                 &osp, &ospsize, data);
+                 (void **) &osp, &ospsize, data);
   if (err)
     {
       zfs_unmount (data);
@@ -3697,8 +3697,7 @@ zfs_mount (grub_device_t dev)
     }
 
   if (ub->ub_version >= SPA_VERSION_FEATURES &&
-      check_mos_features(&((objset_phys_t *) osp)->os_meta_dnode,ub_endian,
-                        data) != 0)
+      check_mos_features(&osp->os_meta_dnode, ub_endian, data) != 0)
     {
       grub_error (GRUB_ERR_BAD_FS, "Unsupported features in pool");
       grub_free (osp);
@@ -3707,8 +3706,7 @@ zfs_mount (grub_device_t dev)
     }
 
   /* Got the MOS. Save it at the memory addr MOS. */
-  grub_memmove (&(data->mos.dn), &((objset_phys_t *) osp)->os_meta_dnode,
-               DNODE_SIZE);
+  grub_memmove (&(data->mos.dn), &osp->os_meta_dnode, DNODE_SIZE);
   data->mos.endian = (grub_zfs_to_cpu64 (ub->ub_rootbp.blk_prop,
                                         ub_endian) >> 63) & 1;
   grub_free (osp);
-- 
2.31.1