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From: Kees Cook <keescook@chromium.org>
Date: Thu, 11 Jan 2018 15:05:14 -0800
Message-ID: <CAGXu5jJ6=XXuCD7zm8NuKAy+-Ra3F2SyD+CVFjJ=-kQoXPty-A@mail.gmail.com>
Subject: Re: [PATCH 13/38] ext4: Define usercopy region in ext4_inode_cache
 slab cache
To: "Theodore Ts'o" <tytso@mit.edu>, Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Windsor <dave@nullcore.net>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Christoph Lameter <cl@linux.com>,
        "David S. Miller" <davem@davemloft.net>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Dave Kleikamp <dave.kleikamp@oracle.com>,
        Jan Kara <jack@suse.cz>,
        Luis de Bethencourt <luisbg@kernel.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Rik van Riel <riel@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel-hardening@lists.openwall.com
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, Jan 11, 2018 at 9:01 AM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Wed, Jan 10, 2018 at 06:02:45PM -0800, Kees Cook wrote:
>> The ext4 symlink pathnames, stored in struct ext4_inode_info.i_data
>> and therefore contained in the ext4_inode_cache slab cache, need
>> to be copied to/from userspace.
>
> Symlink operations to/from userspace aren't common or in the hot path,
> and when they are in i_data, limited to at most 60 bytes.  Is it worth
> it to copy through a bounce buffer so as to disallow any usercopies
> into struct ext4_inode_info?

If this is the only place it's exposed, yeah, that might be a way to
avoid the per-FS patches. This would, AIUI, require changing
readlink_copy() to include a bounce buffer, and that would require an
allocation. I kind of prefer just leaving the per-FS whitelists, as
then there's no global overhead added.

-Kees

-- 
Kees Cook
Pixel Security