linux-kernel - Re: [PATCH] x86/uaccess: small optimization in unsafe_copy_to

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Date:   Sat, 17 Apr 2021 09:27:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user()

On Sat, Apr 17, 2021 at 9:08 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Side note: I'm, looking at the readdir cases that I wrote, and I have
> to just say that is broken too. So "stones and glass houses" etc, and
> I'll have to fix that too.

In particular, the very very old OLD_READDIR interface that only fills
in one dirent at a time didn't call verify_dirent_name(). Same for the
compat version.

This requires a corrupt filesystem to be an issue (and even then,
most/all would have the length of a directory entry in an 'unsigned
char', so even corrupt filesystems would generally never have a
negative name length).

So I don't think it's an issue in _practice_, but at the same time it
is very much an example of the same issue that put_cmsg() has in
net-next: unsafe user copies should be fully guarded and not have some
"but this would never happen because callers would never do anything
bad".

Al - fairly trivial patch applied, comments?

             Linus

View attachment "patch.diff" of type "text/x-patch" (833 bytes)