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

Stefan Berger
2024-01-31T17:27:26Z

On 1/31/24 10:54, Amir Goldstein wrote:
> On Wed, Jan 31, 2024 at 4:40 PM Stefan Berger <stefanb@linux.ibm.com> wrote:
>>
>>
>>
>> On 1/31/24 08:16, Amir Goldstein wrote:
>>> On Wed, Jan 31, 2024 at 4:11 AM Stefan Berger <stefanb@linux.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@linux.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.

I have testcases that work fine with this series when
CONFIG_OVERLAY_FS_METACOPY is not active. Once I activate this then a
test case that changes a file's gid on the overlay layer from 0 to '12'
while causing a copy-up allows a file to execute even thugh it should
not execute. The reason is because d_real_inode(dentry)->i_guid shows
the '0' while d_backing_dentry(dentry)->i_guid shows '12'. User space
stat also shows '12' as expected.

Just saw your other email, will try that now ...