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

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Date: Wed, 31 Jan 2024 09:56:25 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Christian Brauner <brauner@...nel.org>,
        Amir Goldstein <amir73il@...il.com>
Cc: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com,
        zohar@...ux.ibm.com, roberto.sassu@...wei.com, miklos@...redi.hu
Subject: Re: [PATCH 1/5] security: allow finer granularity in permitting
 copy-up of security xattrs



On 1/31/24 09:25, Christian Brauner wrote:
> On Wed, Jan 31, 2024 at 03:25:29PM +0200, Amir Goldstein wrote:
>> On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@...ux.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@...ux.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);
>>
>> and then later...
>>
>>                       error = security_inode_copy_up_xattr(name, value, size);
>>
>> 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?
> 
> So it's relevant if they interact with xattrs that care about the
> idmapping and that's POSIX ACLs and fscaps. And only if they perform
> permission checks such as posix_acl_update_mode() or something. IOW, it
> depends on what exactly EVM is doing.

In 2/5 we are reading the value of security.evm to look at its contents.

> 
> IIRC, I already added custom security_*() hooks specifically for POSIX
> ACLs as they can't be retrieved through vfs_xattr*() helpers anymore.