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

From: Mimi Zohar <zohar@linux.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: Roberto Sassu <roberto.sassu@huawei.com>,
	"linux-integrity@vger.kernel.org"
	<linux-integrity@vger.kernel.org>,
	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Silviu Vlasceanu <Silviu.Vlasceanu@huawei.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [RESEND][PATCH] ima: Set and clear FMODE_CAN_READ in ima_calc_file_hash()
Date: Mon, 16 Nov 2020 13:35:11 -0500	[thread overview]
Message-ID: <3f8cc7c9462353ac2eef58e39beee079bdd9c7b4.camel@linux.ibm.com> (raw)
In-Reply-To: <CAHk-=wjd0RNthZQTLVsnK_d9SFYH0rug2tkezLLB0J-YZzVC+Q@mail.gmail.com>

On Mon, 2020-11-16 at 10:09 -0800, Linus Torvalds wrote:
> On Mon, Nov 16, 2020 at 9:41 AM Christoph Hellwig <hch@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