Re: [PATCH 13/38] ext4: Define usercopy region in ext4_inode_cache slab cache

[Matthew Wilcox]

On Thu, Jan 11, 2018 at 03:05:14PM -0800, Kees Cook wrote:
> On Thu, Jan 11, 2018 at 9:01 AM, Theodore Ts'o <tytso@xxxxxxx> 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.

I think Ted was proposing having a per-FS patch that would, say, copy
up to 60 bytes to the stack, then memcpy it into the ext4_inode_info.