Re: [PATCH] staging: luster: llite: fix a potential missing-check bug when copying lumv

From: Dilger, Andreas
Date: Sat Apr 28 2018 - 12:04:36 EST

On Apr 27, 2018, at 17:45, Wenwen Wang <wang6495@xxxxxxx> wrote:
> [PATCH] staging: luster: llite: fix potential missing-check bug when copying lumv

(typo) s/luster/lustre/

> In ll_dir_ioctl(), the object lumv3 is firstly copied from the user space
> using Its address, i.e., lumv1 = &lumv3. If the lmm_magic field of lumv3 is
> LOV_USER_MAGIV_V3, lumv3 will be modified by the second copy from the user

(typo) s/MAGIV/MAGIC/

> space. The second copy is necessary, because the two versions (i.e.,
> lov_user_md_v1 and lov_user_md_v3) have different data formats and lengths.
> However, given that the user data resides in the user space, a malicious
> user-space process can race to change the data between the two copies. By
> doing so, the attacker can provide a data with an inconsistent version,
> e.g., v1 version + v3 data. This can lead to logical errors in the
> following execution in ll_dir_setstripe(), which performs different actions
> according to the version specified by the field lmm_magic.

This isn't a serious bug in the end. The LOV_USER_MAGIC_V3 check just copies
a bit more data from userspace (the lmm_pool field). It would be more of a
problem if the reverse was possible (copy smaller V1 buffer, but change the
magic to LOV_USER_MAGIC_V3 afterward), but this isn't possible since the second
copy is not done if there is a V1 magic. If the user changes from V3 magic
to V1 in a racy manner it means less data will be used than copied, which
is harmless.

> This patch rechecks the version field lmm_magic in the second copy. If the
> version is not as expected, i.e., LOV_USER_MAGIC_V3, an error code will be
> returned: -EINVAL.

This isn't a bad idea in any case, since it verifies the data copied from
userspace is still valid.

Cheers, Andreas

> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
> ---
> drivers/staging/lustre/lustre/llite/dir.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/drivers/staging/lustre/lustre/llite/dir.c b/drivers/staging/lustre/lustre/llite/dir.c
> index d10d272..80d44ca 100644
> --- a/drivers/staging/lustre/lustre/llite/dir.c
> +++ b/drivers/staging/lustre/lustre/llite/dir.c
> @@ -1185,6 +1185,8 @@ static long ll_dir_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> if (lumv1->lmm_magic == LOV_USER_MAGIC_V3) {
> if (copy_from_user(&lumv3, lumv3p, sizeof(lumv3)))
> return -EFAULT;
> + if (lumv3.lmm_magic != LOV_USER_MAGIC_V3)
> + return -EINVAL;
> }
> if (is_root_inode(inode))
> --
> 2.7.4

Cheers, Andreas
Andreas Dilger
Lustre Principal Architect
Intel Corporation