Re: [PATCH] ovl: skip getxattr of security labels

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Dec 19, 2020 at 12:16 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> When inode has no listxattr op of its own (e.g. squashfs) vfs_listxattr
> calls the LSM inode_listsecurity hooks to list the xattrs that LSMs will
> intercept in inode_getxattr hooks.
>
> When selinux LSM is installed but not initialized, it will list the
> security.selinux xattr in inode_listsecurity, but will not intercept it
> in inode_getxattr.  This results in -ENODATA for a getxattr call for an
> xattr returned by listxattr.
>
> This situation was manifested as overlayfs failure to copy up lower
> files from squashfs when selinux is built-in but not initialized,
> because ovl_copy_xattr() iterates the lower inode xattrs by
> vfs_listxattr() and vfs_getxattr().
>
> ovl_copy_xattr() skips copy up of security labels that are indentified by
> inode_copy_up_xattr LSM hooks, but it does that after vfs_getxattr().
> Since we are not going to copy them, skip vfs_getxattr() of the security
> labels.
>
> Reported-by: Michael Labriola <michael.d.labriola@gmail.com>
> Tested-by: Michael Labriola <michael.d.labriola@gmail.com>
> Link: https://lore.kernel.org/linux-unionfs/2nv9d47zt7.fsf@aldarion.sourceruckus.org/
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>
> Miklos,
>
> This is a workaround for a v5.9 selinux related regression reported by
> Michael that caused copy up failure is a very specific configuration
> involving lower squashfs and built-in but disabled selinux.
>
> I've sent the bug fix to selinux list, so this patch is complementary.
> I removed the stable/Fixes tags, because this patch does not cleanly
> apply to v5.9 and is not the real bug fix.
>

Ping.

FWIW, the selinux bug fix should already be in next.

Thanks,
Amir.

>
>  fs/overlayfs/copy_up.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index e5b616c93e11..0fed532efa68 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -84,6 +84,14 @@ int ovl_copy_xattr(struct super_block *sb, struct dentry *old,
>
>                 if (ovl_is_private_xattr(sb, name))
>                         continue;
> +
> +               error = security_inode_copy_up_xattr(name);
> +               if (error < 0 && error != -EOPNOTSUPP)
> +                       break;
> +               if (error == 1) {
> +                       error = 0;
> +                       continue; /* Discard */
> +               }
>  retry:
>                 size = vfs_getxattr(old, name, value, value_size);
>                 if (size == -ERANGE)
> @@ -107,13 +115,6 @@ int ovl_copy_xattr(struct super_block *sb, struct dentry *old,
>                         goto retry;
>                 }
>
> -               error = security_inode_copy_up_xattr(name);
> -               if (error < 0 && error != -EOPNOTSUPP)
> -                       break;
> -               if (error == 1) {
> -                       error = 0;
> -                       continue; /* Discard */
> -               }
>                 error = vfs_setxattr(new, name, value, size, 0);
>                 if (error) {
>                         if (error != -EOPNOTSUPP || ovl_must_copy_xattr(name))
> --
> 2.25.1
>