[RESEND][PATCH] ima: Set and clear FMODE_CAN_READ in ima_calc_file_hash()

[Mimi Zohar]

On Mon, 2020-11-16 at 10:09 -0800, Linus Torvalds wrote:
> On Mon, Nov 16, 2020 at 9:41 AM Christoph Hellwig <hch at infradead.org> wrote:
> >
> > The "issue" with IMA is that it uses security hooks to hook into the
> > VFS and then wants to read every file that gets opened on a real file
> > system to "measure" the contents vs a hash stashed away somewhere.
> 
> Well, but that's easy enough to handle: if the open isn't a read open,
> then the old contents don't matter, so you shouldn't bother to measure
> the file.
> 
> So this literally sounds like a "doctor, doctor, it hurts when I hit
> my head with a hammer" situation..

We need to differentiate between signed files, which by definition are
immutable, and those that are mutable.  Appending to a mutable file,
for example, would result in the file hash not being updated. 
Subsequent reads would fail.

Mimi