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Date: Thu, 11 Jan 2018 15:05:14 -0800
From: Kees Cook <>
To: "Theodore Ts'o" <>, Kees Cook <>, 
	LKML <>, David Windsor <>, 
	Andreas Dilger <>, 
	Ext4 Developers List <>, Linus Torvalds <>, 
	Alexander Viro <>, Andrew Morton <>, 
	Andy Lutomirski <>, Christoph Hellwig <>, Christoph Lameter <>, 
	"David S. Miller" <>, Laura Abbott <>, 
	Mark Rutland <>, "Martin K. Petersen" <>, 
	Paolo Bonzini <>, Christian Borntraeger <>, 
	Christoffer Dall <>, Dave Kleikamp <>, 
	Jan Kara <>, Luis de Bethencourt <>, Marc Zyngier <>, 
	Rik van Riel <>, Matthew Garrett <>, 
	"" <>, linux-arch <>, 
	Network Development <>, Linux-MM <>,
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 <> 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 Cook
Pixel Security

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