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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 17 Nov 2020 10:23:58 -0800
Message-ID: <CAHk-=wih-ibNUxeiKpuKrw3Rd2=QEAZ8zgRWt_CORAjbZykRWQ@mail.gmail.com>
Subject: Re: [RESEND][PATCH] ima: Set and clear FMODE_CAN_READ in ima_calc_file_hash()
To:     Mimi Zohar <zohar@linux.ibm.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        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>
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On Mon, Nov 16, 2020 at 10:35 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> 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.

Why would that require any reading of the file at all AT WRITE TIME?

Don't do it. Really.

When opening the file write-only, you just invalidate the hash. It
doesn't matter anyway - you're only writing.

Later on, when reading, only at that point does the hash matter, and
then you can do the verification.

Although honestly, I don't even see the point. You know the hash won't
match, if you wrote to the file.

           Linus