Re: [PATCH] mm/shmem.c: don't set 'seals' to 'F_SEAL_SEAL' in shmem_get_inode

[yukuai (C)]

On 2019/11/27 12:24, Hugh Dickins Wrote:
> On Wed, 27 Nov 2019, yu kuai wrote:
>
> > 'seals' is set to 'F_SEAL_SEAL' in shmem_get_inode, which means "prevent
> > further seals from being set", thus sealing API will be useless and many
> > code in shmem.c will never be reached. For example:
>
> The sealing API is not useless, and that code can be reached.
>
> > shmem_setattr
> > 	if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
> > 	    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> > 		return -EPERM;
> >
> > So, initialize 'seals' to zero is more reasonable.
> >
> > Signed-off-by: yu kuai <yukuai3@xxxxxxxxxx>
>
> NAK.
>
> See memfd_create in mm/memfd.c (code which originated in mm/shmem.c,
> then was extended to support hugetlbfs also): sealing is for memfds,
> not for tmpfs or hugetlbfs files or SHM.  Without thinking about it too
> hard, I believe that to allow sealing on tmpfs files would introduce
> surprising new behaviors on them, which might well raise security issues;
> and also be incompatible with the guarantees intended by sealing.

Thank you for your response.
Yu Kuai