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

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

On 12/11/2025 6:50 AM, Roberto Sassu wrote:
> On Thu, 2025-12-11 at 10:56 +0100, Roberto Sassu wrote:
>> On Wed, 2025-12-10 at 11:12 -0800, Gregory Lumen wrote:
>>> Roberto,
>>>
>>> The proposed approach appears to be workable. However, if our primary goal
>>> here is to enable UM to free kernel memory consumed by the IMA log with an
>>> absolute minimum of kernel functionality/change, then I would argue that
>>> the proposed Stage-then-delete approach still represents unnecessary
>>> complexity when compared to a trim-to-N solution. Specifically:
> The benefit of the Stage-then-delete is that you don't need to scan the
> IMA measurements list in advance to determine what to trim, you just
> trim everything by swapping list head (very fast) and then you can read
> and delete the measurements out of the hot path.
>
> [...]

Trim N entries proposal can do the same speed as staged proposal or 
faster than staged proposal.

Steven

>>> - There exists a potential UM measurement-loss race condition introduced
>>> by the staging functionality that would not exist with a trim-to-N
>>> approach. (Occurs if a kexec call occurs after a UM agent has staged
>>> measurements for deletion, but has not completed copying them to
>>> userspace). This could be avoided by persisting staged measurements across
>>> kexec calls at the cost of making the proposed change larger.
>> The solution is to coordinate the staging with kexec in user space.
> To avoid requiring coordination in user space, I will try to see if I
> could improve my patch to prepend the staged entries to the current
> measurement list, before serializing them for kexec().

Trim N entries proposal does not need this at all.

Steven

> Roberto