Re: [PATCH] hibernate: unlock swap bdev for writing when uswsusp is active

[Domenico Andreoli <domenico.andreoli@linux.com>]

On Sat, Feb 29, 2020 at 10:38:20AM -0800, Darrick J. Wong wrote:
> On Sat, Feb 29, 2020 at 07:07:16PM +0100, Domenico Andreoli wrote:
> > On Sat, Feb 29, 2020 at 09:08:25AM -0800, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > It turns out that there /is/ one use case for programs being able to
> > > write to swap devices, and that is the userspace hibernation code.  The
> > > uswsusp ioctls allow userspace to lease parts of swap devices, so turn
> > > S_SWAPFILE off when invoking suspend.
> > > 
> > > Fixes: 1638045c3677 ("mm: set S_SWAPFILE on blockdev swap devices")
> > > Reported-by: Domenico Andreoli <domenico.andreoli@linux.com>
> > > Reported-by: Marian Klein <mkleinsoft@gmail.com>
> > 
> > I also tested it yesterday but was not satisfied, unfortunately I did
> > not come with my comment in time.
> > 
> > Yes, I confirm that the uswsusp works again but also checked that
> > swap_relockall() is not triggered at all and therefore after the first
> > hibernation cycle the S_SWAPFILE bit remains cleared and the whole
> > swap_relockall() is useless.
> > 
> > I'm not sure this patch should be merged in the current form.
> 
> NNGGHHGGHGH /me is rapidly losing his sanity and will soon just revert
> the whole security feature because I'm getting fed up with people
> yelling at me *while I'm on vacation* trying to *restore* my sanity.  I
> really don't want to be QAing userspace-directed hibernation right now.

Maybe we could proceed with the first patch to amend the regression and
postpone the improved fix to a later patch? Don't loose sanity for this.

> ...right, the patch is broken because we have to relock the swapfiles in
> whatever code executes after we jump back to the restored kernel, not in
> the one that's doing the restoring.  Does this help?

I made a few unsuccessful attempts in kernel/power/hibernate.c and
eventually I'm switching to qemu to speed up the test cycle.

> OTOH, maybe we should just leave the swapfiles unlocked after resume.
> Userspace has clearly demonstrated the one usecase for writing to the
> swapfile, which means anyone could have jumped in while uswsusp was
> running and written whatever crap they wanted to the parts of the swap
> file that weren't leased for the hibernate image.

Essentially, if the hibernation is supported the swapfile is not totally
safe. Maybe user-space hibernation should be a separate option.

> 
> --D

-- 
rsa4096: 3B10 0CA1 8674 ACBA B4FE  FCD2 CE5B CF17 9960 DE13
ed25519: FFB4 0CC3 7F2E 091D F7DA  356E CC79 2832 ED38 CB05