Re: [PATCH 5.10 036/103] ucounts: Increase ucounts reference counter before the security hook

[Greg Kroah-Hartman]

On Thu, Sep 02, 2021 at 01:06:34PM -0500, Eric W. Biederman wrote:
> Sasha Levin <sashal@xxxxxxxxxx> writes:
> 
> > On Wed, Sep 01, 2021 at 12:26:10PM -0500, Eric W. Biederman wrote:
> >>Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> >>
> >>> On Wed, Sep 01, 2021 at 09:25:25AM -0500, Eric W. Biederman wrote:
> >>>> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> >>>>
> >>>> > From: Alexey Gladkov <legion@xxxxxxxxxx>
> >>>> >
> >>>> > [ Upstream commit bbb6d0f3e1feb43d663af089c7dedb23be6a04fb ]
> >>>> >
> >>>> > We need to increment the ucounts reference counter befor security_prepare_creds()
> >>>> > because this function may fail and abort_creds() will try to decrement
> >>>> > this reference.
> >>>>
> >>>> Has the conversion of the rlimits to ucounts been backported?
> >>>>
> >>>> Semantically the code is an improvement but I don't know of any cases
> >>>> where it makes enough of a real-world difference to make it worth
> >>>> backporting the code.
> >>>>
> >>>> Certainly the ucount/rlimit conversions do not meet the historical
> >>>> criteria for backports.  AKA simple obviously correct patches.
> >>>>
> >>>> The fact we have been applying fixes for the entire v5.14 stabilization
> >>>> period is a testament to the code not quite being obviously correct.
> >>>>
> >>>> Without backports the code only affects v5.14 so I have not been
> >>>> including a Cc stable on any of the commits.
> >>>>
> >>>> So color me very puzzled about what is going on here.
> >>>
> >>> Sasha picked this for some reason, but if you think it should be
> >>> dropped, we can easily do so.
> >>
> >>My question is what is the reason Sasha picked this up?
> >>
> >>If this patch even applies to v5.10 the earlier patches have been
> >>backported.  So we can't just drop this patch.  Either the earlier
> >>backports need to be reverted, or we need to make certain all of the
> >>patches are backported.
> >>
> >>I really am trying to understand what is going on and why.
> >
> > I'll happily explain. The commit message is telling us that:
> >
> > 1. There is an issue uncovered by syzbot which this patch fixes:
> >
> > 	"Reported-by: syzbot"
> >
> > 2. The issue was introduced in 905ae01c4ae2 ("Add a reference to ucounts
> > for each cred"):
> >
> > 	"Fixes: 905ae01c4ae2"
> >
> > Since 905ae01c4ae2 exist in 5.10, and this patch seemed to fix an issue,
> > I've queued it up.
> 
> Which begs the question as Alex mentioned how did 905ae01c4ae2 get into
> 5.10, as it was merged to Linus's tree in the merge window for 5.14.
> 
> > In general, if we're missing backports, backported something only
> > partially and should revert it, or anything else that might cause an
> > issue, we'd be more than happy to work with you to fix it up.
> >
> > All the patches we queue up get multiple rounds of emails and reviews,
> > if there is a better way to solicit reviews so that we won't up in a
> > place where you haven't noticed something going in earlier we'd be more
> > than happy to improve that process too.
> 
> I have the bad feeling that 905ae01c4ae2 was backported because it was a
> prerequisite to something with a Fixes tag.
> 
> Fixes tags especially in this instance don't mean code needs to go to
> stable Fixes tags mean that a bug was fixed.  Since I thought the code
> only existed in Linus's tree, I haven't been adding Cc stable or even
> thinking about earlier kernels with respect to this code.
> 
> I honestly can't keep up with the level of review needed for patches
> targeting Linus's tree.  So I occasionally glance at patches destined
> for the stable tree.
> 
> Most of the time it is something being backported without a stable tag,
> but with a fixes tag, that is unnecessary but generally harmless so I
> ignore it.
> 
> In this instance it looks like a whole new feature that has had a rocky
> history and a lot of time to stablize is somehow backported to 5.10 and
> 5.13.  I think all of the known issues are addressed but I won't know
> if all of the issues syzkaller can find are found for another couple of
> weeks.
> 
> Because this code was not obviously correct, because this code did not
> have a stable tag, because I am not even certain it is stable yet,
> I am asking do you know how this code that feels to me like feature work
> wound up being backported?  AKA why is 905ae01c4ae2 in 5.10 and 5.13.

Looks like Sasha added it to the tree last week and it went out in the
last set of releases.  Sasha, why was this added?  Let me see if it was
a requirement of some other patch...

thanks,

greg k-h