Re: [PATCH 0/2] ima: policy search speedup

[Kasatkin, Dmitry]

On Tue, Dec 11, 2012 at 8:35 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Dec 11, 2012 at 10:12 AM, Kasatkin, Dmitry
> <dmitry.kasatkin@xxxxxxxxx> wrote:
>>
>> Actually S_PRIVATE does not work work for normal filesystems which IMA
>> might want to ignore.
>
> The reading comprehension here is abysmal.
>
> First you claim that you need the new flag for pseudo-filesystems, and
> now that I point out that we have an *old* flag for pseudo-filesystems
> you turn around 180 degrees and talk about other filesystems.
>

I have not claimed that. This is exactly what I have written:
"There are different filesystems which are not checked by IMA/EVM,
such as pseudo-filesystems"
Pseudo-filesystems was an example of possible cases.
Sorry if it was not clear enough.

> And none of that matters for my argument AT ALL.
>
> My argument has not been that we cannot add a new flag.
>
> My argument has been that we already have the logical place for such a
> flag, and that adding a totally new field seems so stupid.
>
> Seriously. The i_flags place is where we already do pretty much
> *exactly* what you ask for. The fact that it is faster and more
> flexible to boot should be a bonus.
>
> Now, there are real reasons to avoid "s_flags", notably the fact that
> we're running out of bits there (unlike i_flags), and they are exposed
> as generic fields and are generally meant for mount options etc. So I
> understand why we might want to avoid that (although the whole
> mount-option thing could also be seen as an advantage), but I really
> don't see any argument against i_flags, considering that we already
> use it for S_IMA and S_PRIVATE, both of which are related to exactly
> what you seem to want to do.

Not every inode on the filesystem might be checked by IMA.
It depends on policy. And S_IMA flag is used exactly for this purpose.

But when the entire FS is not checked, SB flag seems to be very appropriate.
Eric has given nice explanation.

>
> The one downside of i_flags may be that any update should own the
> inode semaphore. But within the context of a security model, that
> should be fine (and normally you'd update it once per lifetime of the
> inode).
>

This is exactly how IMA works at the moment.
See my response to Eric about performance.

>                   Linus
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