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Date: Thu, 11 Jan 2018 15:05:14 -0800
From: Kees Cook <keescook@...omium.org>
To: "Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>, 
	LKML <linux-kernel@...r.kernel.org>, David Windsor <dave@...lcore.net>, 
	Andreas Dilger <adilger.kernel@...ger.ca>, 
	Ext4 Developers List <linux-ext4@...r.kernel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Lutomirski <luto@...nel.org>, Christoph Hellwig <hch@...radead.org>, Christoph Lameter <cl@...ux.com>, 
	"David S. Miller" <davem@...emloft.net>, Laura Abbott <labbott@...hat.com>, 
	Mark Rutland <mark.rutland@....com>, "Martin K. Petersen" <martin.petersen@...cle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ibm.com>, 
	Christoffer Dall <christoffer.dall@...aro.org>, Dave Kleikamp <dave.kleikamp@...cle.com>, 
	Jan Kara <jack@...e.cz>, Luis de Bethencourt <luisbg@...nel.org>, Marc Zyngier <marc.zyngier@....com>, 
	Rik van Riel <riel@...hat.com>, Matthew Garrett <mjg59@...gle.com>, 
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, 
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 13/38] ext4: Define usercopy region in ext4_inode_cache
 slab cache

On Thu, Jan 11, 2018 at 9:01 AM, Theodore Ts'o <tytso@....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

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