Re: [RFC][PATCH v2] ima: Add support for staging measurements for deletion and trimming

[steven chen <chenste@linux.microsoft.com>]

On 1/29/2026 12:20 AM, Roberto Sassu wrote:
> On 1/28/2026 10:30 PM, steven chen wrote:
>> On 12/12/2025 9:19 AM, Roberto Sassu wrote:
>>> From: Roberto Sassu <roberto.sassu@huawei.com>
>>>
>>> Introduce the ability of staging the entire (or a portion of the) IMA
>>> measurement list for deletion. Staging means moving the current 
>>> content of
>>> the measurement list to a separate location, and allowing users to 
>>> read and
>>> delete it. This causes the measurement list to be atomically truncated
>>> before new measurements can be added. Staging can be done only once 
>>> at a
>>> time. In the event of kexec(), staging is reverted and staged 
>>> entries will
>>> be carried over to the new kernel.
>>>
>>> User space is responsible to concatenate the staged IMA measurements 
>>> list
>>> portions following the temporal order in which the operations were 
>>> done,
>>> together with the current measurement list. Then, it can send the 
>>> collected
>>> data to the remote verifiers.
>>>
>>> Also introduce the ability of trimming N measurements entries from 
>>> the IMA
>>> measurements list, provided that user space has already read them. 
>>> Trimming
>>> combines staging and deletion in one operation.
>>>
>>> The benefit of these solutions is the ability to free precious kernel
>>> memory, in exchange of delegating user space to reconstruct the full
>>> measurement list from the chunks. No trust needs to be given to user 
>>> space,
>>> since the integrity of the measurement list is protected by the TPM.
>>>
>>> By default, staging/trimming the measurements list does not alter 
>>> the hash
>>> table. When staging/trimming are done, IMA is still able to detect
>>> collisions on the staged and later deleted measurement entries, by 
>>> keeping
>>> the entry digests (only template data are freed).
>>>
>>> However, since during the measurements list serialization only the SHA1
>>> digest is passed, and since there are no template data to 
>>> recalculate the
>>> other digests from, the hash table is currently not populated with 
>>> digests
>>> from staged/deleted entries after kexec().
>>>
>>> Introduce the new kernel option ima_flush_htable to decide whether 
>>> or not
>>> the digests of staged measurement entries are flushed from the hash 
>>> table.
>>>
>>> Then, introduce ascii_runtime_measurements_staged_<algo> and
>>> binary_runtime_measurement_staged_<algo> interfaces to 
>>> stage/trim/delete
>>> the measurements. Use 'echo A > <IMA interface>' and
>>> 'echo D > <IMA interface>' to respectively stage and delete the entire
>>> measurements list. Use 'echo N > <IMA interface>', with N between 1 and
>>> LONG_MAX, to stage the selected portion of the measurements list, and
>>> 'echo -N > <IMA interface>' to trim N measurements entries.
>>>
>>> The ima_measure_users counter (protected by the ima_measure_lock 
>>> mutex) has
>>> been introduced to protect access to the measurements list and the 
>>> staged
>>> part. The open method of all the measurement interfaces has been 
>>> extended
>>> to allow only one writer at a time or, in alternative, multiple 
>>> readers.
>>> The write permission is used to stage/trim/delete the measurements, the
>>> read permission to read them. Write requires also the CAP_SYS_ADMIN
>>> capability.
>>>
>>> Finally, introduce and maintain dedicate counters for the number of
>>> measurement entries and binary size, for the current measurements list
>>> (BINARY_SIZE), for the current measurements list plus staged entries
>>> (BINARY_SIZE_STAGED) useful for kexec() segment allocation, and for the
>>> entire measurement list without staging/trimming (BINARY_SIZE_FULL) 
>>> useful
>>> for the kexec-related critical data records.
>> Is the following possible race condition for staged list:
>>
>> Agent A: create staged list            Staged list A1
>>           new measurement added    Measurement list M1
>>           Two lists in kernel: A1 and M1
>>
>> Agent B: read staged list (A1) to do verification
>>           new measurement added    Measurement list M2
>>           Two lists in kernel: A1 and M2
>>
>> Agent A: verified and remove staged list (A1)
>>           new measurement added    Measurement list M3
>>           One list in kernel: M3
>>
>> Agent C: create staged list            Staged list C1
>>           new measurement added    Measurement list M4
>>           Two lists in kernel: C1 and M4
>>
>> Agent B: remove staged list (?), C1 removed ---this will cause problem
>>           new measurement added    Measurement list M5
>>           One list in kernel: M5
>>
>> Agent C: try to remove staged list(?)
>
> If you remember the patch, we added a read-write protection to the 
> measurements interfaces. As long as you keep the interface open for 
> write no one else can make change on the staging. Sure, you can drop 
> the write, and reopen for read, but then you should expect someone 
> else to operate on the interface.
>
> If you want to be sure no one else changes the staged measurements, 
> just keep the interface open for write, read the staged measurements 
> and delete them.
>
> Roberto
>
For different use cases, we can compare lock time for both staged method 
and trim N method:

t1: user space measurement list lock time
t2: kernel measurement list lock time

     Stage approach use case 1:
               1. read PCR quote
               2. read list
               3. attestation
               4. get N from attestation response
---          5. hold the list in the user space
  ^   ---    6. hold the measurement list
        ^     7. stage the list
t1    t2   8. trim N
        v     9. put the rest of stage back to measurement list
  v   ---    10. release the measurement list
---          11. release the list in the user space
  For this case, agent race condition may happen

   Stage approach use case 2:
               1. read PCR quote
---          2. hold the list in the user space
  ^           3. stage the list
               4. read list
               5. attestation
t1    ---  6. hold the measurement list
          ^   7. get N from attestation response
          t2  8. trim N
          v    9. put the rest of stage back to measurement list
  v    ---   10. release the measurement list
---          11. release the list in the user space
  For this case, no agent race condition happen

the following use case for trim N method

    Trim N approach use case:
              1. read total trimmed T
              2. read PCR quote
              3. read list,
              4. attestation
              5. get N from attestation response
---         6. hold the list in the user space
  ^   ---   7. hold the measurement list
        ^
t1   t2   8. trim with format T:N, update T
        v
  v   ---    9 . release the measurement list
---          10. release the list in the user space
     no agent race condition happen

For all use cases, I think for both t1 and t2, trim N method has better 
result.

Steven

>> Possible solution?
>>    Save the total number trimmed T or tag
>>
>>    Trim request sync this parameter to trim the staged list
>>
>> Regards,
>>
>> Steven
>>
>>> Note: This code derives from the Alt-IMA Huawei project, and is being
>>>        released under the dual license model (GPL-2.0 OR MIT).
>>>
>>> Link: https://github.com/linux-integrity/linux/issues/1
>>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
>>> ---