Re: [PATCH 1/5] security: allow finer granularity in permitting copy-up of security xattrs

Stefan Berger
2024-02-01T15:41:41Z

On 1/31/24 08:25, Amir Goldstein wrote:
> On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@linux.ibm.com> wrote:
>>
>> Copying up xattrs is solely based on the security xattr name. For finer
>> granularity add a dentry parameter to the security_inode_copy_up_xattr
>> hook definition, allowing decisions to be based on the xattr content as
>> well.
>>
>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>> ---
>> fs/overlayfs/copy_up.c | 2 +-
>> include/linux/evm.h | 2 +-
>> include/linux/lsm_hook_defs.h | 3 ++-
>> include/linux/security.h | 4 ++--
>> security/integrity/evm/evm_main.c | 2 +-
>> security/security.c | 7 ++++---
>> security/selinux/hooks.c | 2 +-
>> security/smack/smack_lsm.c | 2 +-
>> 8 files changed, 13 insertions(+), 11 deletions(-)
>>
>> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
>> index b8e25ca51016..bd9ddcefb7a7 100644
>> --- a/fs/overlayfs/copy_up.c
>> +++ b/fs/overlayfs/copy_up.c
>> @@ -114,7 +114,7 @@ int ovl_copy_xattr(struct super_block *sb, const struct path *oldpath, struct de
>> if (ovl_is_private_xattr(sb, name))
>> continue;
>>
>> - error = security_inode_copy_up_xattr(name);
>> + error = security_inode_copy_up_xattr(old, name);
>
> What do you think about:
>
> error = security_inode_copy_up_xattr(name, NULL, 0);

We need 'old'.
>
> and then later...
>
> error = security_inode_copy_up_xattr(name, value, size);

Are these parameter used to first query for the necessary size of the
buffer and then provide the buffer to fill it? Or should the function
rather take an existing buffer and realloc it if necessary and place the
value of the xattr into it? Unfortunately this function currently
returns '1' for 'discard', so returning the size of the xattr value from
it maybe not ideal but it would require maybe yet another parameter that
indicates what the size of the xattr value is.

Stefan

>
> I am asking because overlayfs uses mnt_idmap(path->mnt) and you
> have used nop_mnt_idmap inside evm hook.
> this does not look right to me?
>
> Thanks,
> Amir.