linux-kernel - Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate metadata hash

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Message-ID: <492ea12a-d79d-47da-9bbe-a7f33051bd3f@linux.ibm.com>
Date: Wed, 31 Jan 2024 12:46:50 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: 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 4/5] evm: Use the real inode's metadata to calculate
 metadata hash



On 1/31/24 12:23, Amir Goldstein wrote:
> On Wed, Jan 31, 2024 at 5:54 PM Amir Goldstein <amir73il@...il.com> wrote:
>>
>> On Wed, Jan 31, 2024 at 4:40 PM Stefan Berger <stefanb@...ux.ibm.com> wrote:
>>>
>>>
>>>
>>> On 1/31/24 08:16, Amir Goldstein wrote:
>>>> On Wed, Jan 31, 2024 at 4:11 AM Stefan Berger <stefanb@...ux.ibm.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 1/30/24 16:46, Stefan Berger wrote:
>>>>>> Changes to the file attribute (mode bits, uid, gid) on the lower layer
>>>>>> are not take into account when d_backing_inode() is used when a file is
>>>>>> accessed on the overlay layer and this file has not yet been copied up.
>>>>>> This is because d_backing_inode() does not return the real inode of the
>>>>>> lower layer but instead returns the backing inode which holds old file
>>>>>> attributes. When the old file attributes are used for calculating the
>>>>>> metadata hash then the expected hash is calculated and the file then
>>>>>> mistakenly passes signature verification. Therefore, use d_real_inode()
>>>>>> which returns the inode of the lower layer for as long as the file has
>>>>>> not been copied up and returns the upper layer's inode otherwise.
>>>>>>
>>>>>> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
>>>>>> ---
>>>>>>     security/integrity/evm/evm_crypto.c | 2 +-
>>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
>>>>>> index b1ffd4cc0b44..2e48fe54e899 100644
>>>>>> --- a/security/integrity/evm/evm_crypto.c
>>>>>> +++ b/security/integrity/evm/evm_crypto.c
>>>>>> @@ -223,7 +223,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
>>>>>>                                  size_t req_xattr_value_len,
>>>>>>                                  uint8_t type, struct evm_digest *data)
>>>>>>     {
>>>>>> -     struct inode *inode = d_backing_inode(dentry);
>>>>>> +     struct inode *inode = d_real_inode(dentry);
>>>>>>         struct xattr_list *xattr;
>>>>>>         struct shash_desc *desc;
>>>>>>         size_t xattr_size = 0;
>>>>>
>>>>> We need this patch when NOT activating CONFIG_OVERLAY_FS_METACOPY but
>>>>> when setting CONFIG_OVERLAY_FS_METACOPY=y it has to be reverted...  I am
>>>>> not sure what the solution is.
>>>>
>>>> I think d_real_inode() does not work correctly for all its current users for
>>>> a metacopy file.
>>>>
>>>> I think the solution is to change d_real_inode() to return the data inode
>>>> and add another helper to get the metadata inode if needed.
>>>> I will post some patches for it.
>>>
>>> I thought that we may have to go through vfs_getattr() but even better
>>> if we don't because we don't have the file *file anywhere 'near'.
>>>
>>>>
>>>> However, I must say that I do not know if evm_calc_hmac_or_hash()
>>>> needs the lower data inode, the upper metadata inode or both.
>>>
>>> What it needs are data structures with mode bits, uid, and gid that stat
>>> in userspace would show.
>>>
>>>
>>
>> With or without metacopy enabled, an overlay inode st_uid st_gid st_mode
>> are always taken from the upper most inode which is what d_real_inode()
>> currently returns, so I do not understand what the problem is.
>>
> 
> No, I was wrong. It is the other way around.
> d_real_inode() always returns the real data inode and you need the
> upper most real inode.
> 
> You can try this:
> 
>   -     struct inode *inode = d_backing_inode(dentry);
> +     struct inode *inode = d_inode(d_real(dentry, false));
> 
> With the changes in:
> 
> https://github.com/amir73il/linux/commits/overlayfs-devel/
> 
> Not thoroughly tested...

The change + 3 topmost patches cherry-picked is unfortunately are 
crashing for me.

FYI - in case you are interested: My tests running this with UML are here:

repo: https://github.com/stefanberger/ima-namespaces-tests.git
branch: overlayfs

There's a UML config in config/config.uml
Compiler kernel with this series applied: make ARCH=um -j128  && yes | 
cp linux /usr/local/bin/linux

sudo IMA_TEST_UML=/usr/local/bin/linux IMA_TEST_VERBOSE=0 
evm+overlayfs-1/test.sh
sudo IMA_TEST_UML=/usr/local/bin/linux IMA_TEST_VERBOSE=0 
evm+overlayfs-2/test.sh
sudo IMA_TEST_UML=/usr/local/bin/linux IMA_TEST_VERBOSE=0 
evm+overlayfs-3/test.sh

The 2nd and 3rd test case will fail at some point when metacopy is 
enabled, otherwise they will all pass.

> 
> Thanks,
> Amir.
>