Messages in this thread | | | Subject | Re: [PATCH 3/4] integrity: IMA as an integrity service provider | From | Dave Hansen <> | Date | Fri, 21 Nov 2008 09:38:50 -0800 |
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On Thu, 2008-11-20 at 20:39 -0500, Mimi Zohar wrote: > On Thu, 2008-11-20 at 13:22 -0800, Dave Hansen wrote: > > On Thu, 2008-11-20 at 11:43 -0500, Mimi Zohar wrote: > > > > > > + /* Invalidate PCR, if a measured file is already open for read > > > */ > > > + if ((mask == MAY_WRITE) || (mask == MAY_APPEND)) { > > > + int mask_sav = data->mask; > > > + int rc; > > > + > > > + data->mask = MAY_READ; > > > + rc = ima_must_measure(&idata); > > > + if (!rc) { > > > + if (atomic_read(&(data->dentry->d_count)) - 1 > > > > + atomic_read(&(inode->i_writecount))) > > > + ima_add_violation(inode, data->filename, > > > + "invalid_pcr", "ToMToU"); > > > + } > > > + data->mask = mask_sav; > > > + goto out; > > > + } > > > > Following up on Christoph's comment... > > > > I'm worried that this calculation isn't very precise. The calculation > > that you're trying to come up with here is the number of opens (d_count) > > vs. the number of writers (i_writecount). When they don't match, you > > know that the new open is the first write, and you must 'invalidate the > > PCR'? > > > > There are a number of things that elevate d_count, and it is a lot more > > than just an open() that can do it. Is that OK? > > >From an integrity perspective, a file measurement might be invalidated > unnecessarily, but it is safe. For any file when opened for write, while > having an existing reader, will cause the file measurement to be > invalidated. Can you give examples of things, other than open(), that > elevate d_count?
Just do a little search for dget(). There are plenty of places that we'll do a lookup (and a dget), realize that we can't complete an operation (say O_RDWR on a 400 file), then dput() the dentry. If you look at dput at the wrong time, you could mistake that for an *actual* open.
Are you holding any locks here? If not, it is completely conceivable that you do the atomic_read(d_count)=100, then by the time you do the aomtic_read(i_writecount), 90 of those references have been dput()'d.
> Is there a different, better way to determine if there are any readers?
I think you're looking at it from the wrong angle. If you have a writer, does it matter whether there are any readers? You should just unconditionally invalidate the integrity measurement.
-- Dave
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