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

[David Sterba]

On Wed, Feb 17, 2021 at 09:04:34AM +0300, Dan Carpenter wrote:
> 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 
> ---
> Presumably only root can create snapshots.

Well, no. After first analysis there are some "interesting memory access
patterns" possible, with a crafted data in the inherit member.

> Anyway, I have not tested
> this fix.  I believe it is correct, of course.  But perhaps it's best
> to check.

Yeah I'll write a test also to see where exactly the issues are. Thanks
for the report/fix.

[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 
---
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