Re: Security fixes for 4.4

[Ben Hutchings <ben.hutchings@codethink.co.uk>]

On Thu, 2017-11-16 at 17:07 -0500, Theodore Ts'o wrote:
> On Thu, Nov 16, 2017 at 12:29:56PM +0100, Greg Kroah-Hartman wrote:
> > On Wed, Nov 15, 2017 at 09:10:46PM +0000, Ben Hutchings wrote:
> > > Please apply the attached backported patches to 4.4-stable.  The
> > > upstream commits are:
> > > 
> > > 06bd3c36a733 ext4: fix data exposure after a crash
> > 
> > This patch did not apply, and when I worked at it by hand to apply, it
> > then broke the build with:
> > 	fs/ext4/inode.c: In function ‘ext4_map_blocks’:
> > fs/ext4/inode.c:669:17: error: ‘EXT4_GET_BLOCKS_ZERO’ undeclared (first use in this function); did you mean ‘EXT4_GET_BLOCKS_PRE_IO’?
> >        !(flags & EXT4_GET_BLOCKS_ZERO) &&
> >                  ^~~~~~~~~~~~~~~~~~~~
> > 
> > As Ted didn't provide this on the list of ext4 patches to backport to
> > 4.4 in the past, I'm a bit hesitant to take this now.  Are you sure it
> > is needed?
> 
> As Greg noted, EXT4_GET_BLOCKS_ZERO is not in the Linux 4.4 kernel.
> To make this works requires at least three pre-requisite commits:
> 
> 2dcba4781fa3: ext4: get rid of EXT4_GET_BLOCKS_NO_LOCK flag
> 53085fac02d1: ext4: provide ext4_issue_zeroout()
> c86d8db33a92: ext4: implement allocation of pre-zeroed blocks
[...]

I previously backported this to 3.16 and simply removed that flag
check, on the basis that the feature it represents is not supported at
all.  The backports to 3.10 and 3.12, and the backport I sent to Greg
for 4.4 (as an attachment), also made that change.  Are you saying that
a backport of the fix actually needs to check for a similar condition,
even in branches where the flag doesn't exist?

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.