[PATCH] btrfs: prevent potential out of bounds in btrfs_ioctl_snap_create_v2()

[Dan Carpenter]

The problem is we're copying "inherit" from user space but we don't
necessarily know that we're copying enough data for a 64 byte
struct.  Then the next problem is that "inherit" has a variable size
array at the end, and we have to verify that array is the size we
expected.

Fixes: 6f72c7e20dba: ("Btrfs: add qgroup inheritance")
Signed-off-by: Dan Carpenter <dan.carpen...@oracle.com>
---
Presumably only root can create snapshots.  Anyway, I have not tested
this fix.  I believe it is correct, of course.  But perhaps it's best
to check.

The calculation for the number of elements in the array was copied from
btrfs_qgroup_inherit().

 fs/btrfs/ioctl.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 384d33ab02c7..9d7b24d3e3bd 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1941,15 +1941,34 @@ static noinline int btrfs_ioctl_snap_create_v2(struct 
file *file,
        if (vol_args->flags & BTRFS_SUBVOL_RDONLY)
                readonly = true;
        if (vol_args->flags & BTRFS_SUBVOL_QGROUP_INHERIT) {
-               if (vol_args->size > PAGE_SIZE) {
+               u64 nums;
+
+               if (vol_args->size < sizeof(*inherit) ||
+                   vol_args->size > PAGE_SIZE) {
                        ret = -EINVAL;
                        goto free_args;
                }
+
                inherit = memdup_user(vol_args->qgroup_inherit, vol_args->size);
                if (IS_ERR(inherit)) {
                        ret = PTR_ERR(inherit);
                        goto free_args;
                }
+
+               /* quick and dirty checks to prevent integer overflows */
+               if (inherit->num_qgroups > PAGE_SIZE ||
+                   inherit->num_ref_copies > PAGE_SIZE ||
+                   inherit->num_excl_copies > PAGE_SIZE) {
+                       ret = -EINVAL;
+                       goto free_inherit;
+               }
+
+               nums = inherit->num_qgroups + 2 * inherit->num_ref_copies +
+                      2 * inherit->num_excl_copies;
+               if (vol_args->size != struct_size(inherit, qgroups, nums)) {
+                       ret = -EINVAL;
+                       goto free_inherit;
+               }
        }
 
        ret = __btrfs_ioctl_snap_create(file, vol_args->name, vol_args->fd,
-- 
2.30.0