Re: please revert backport of 44c76825d6eefee9eb7ce06c38e1a6632ac7eb7d

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Greg,

On Thu, Dec 05, 2024 at 08:07:15AM +0100, Greg KH wrote:
> On Thu, Dec 05, 2024 at 07:54:40AM +0100, Salvatore Bonaccorso wrote:
> > Hi all,
> > 
> > On Mon, Nov 25, 2024 at 10:26:12AM +0300, Michael Tokarev wrote:
> > > 21.11.2024 17:33, Kees Cook wrote:
> > > > Hi stable tree maintainers,
> > > > 
> > > > Please revert the backports of
> > > > 
> > > > 44c76825d6ee ("x86: Increase brk randomness entropy for 64-bit systems")
> > > > 
> > > > namely:
> > > > 
> > > > 5.4:  03475167fda50b8511ef620a27409b08365882e1
> > > > 5.10: 25d31baf922c1ee987efd6fcc9c7d4ab539c66b4
> > > > 5.15: 06cb3463aa58906cfff72877eb7f50cb26e9ca93
> > > > 6.1:  b0cde867b80a5e81fcbc0383e138f5845f2005ee
> > > > 6.6:  1a45994fb218d93dec48a3a86f68283db61e0936
> > > > 
> > > > There seems to be a bad interaction between this change and older
> > > > PIE-built qemu-user-static (for aarch64) binaries[1]. Investigation
> > > > continues to see if this will need to be reverted from 6.6, 6.11,
> > > > and mainline. But for now, it's clearly a problem for older kernels with
> > > > older qemu.
> > > > 
> > > > Thanks!
> > > > 
> > > > -Kees
> > > > 
> > > > [1] https://lore.kernel.org/all/202411201000.F3313C02@keescook/
> > > Unfortunately I haven't seen this thread and this email before now,
> > > when things are already too late.
> > > 
> > > And it turned out it's entirely my fault with all this.  Let me
> > > explain so things become clear to everyone.
> > > 
> > > The problem here is entirely in qemu-user.  The fundamental issue
> > > is that qemu-user does not implement an MMU, instead, it implements
> > > just address shift, searching for a memory region for the guest address
> > > space which is hopefully not used by qemu-user itself.
> > > 
> > > In practice, this is rarely an issue though, when - and this is the
> > > default - qemu is built as a static-pie executable.  This is important:
> > > it's the default mode for the static build - it builds as static-pie
> > > executable, which works around the problem in almost all cases.
> > > This is done for quite a long time, too.
> > > 
> > > However, I, as qemu maintainer in debian, got a bug report saying
> > > that qemu-user-static isn't "static enough" - because for some tools
> > > used on debian (lintian), static-pie was something unknown and the
> > > tool issued a warning.  And at the time, I just added --disable-pie
> > > flag to the build, without much thinking.  This is where things went
> > > wrong.
> > > 
> > > Later I reverted this change with a shame, because it causes numerous
> > > configurations to fail randomly, and each of them is very difficult to
> > > debug (especially due to randomness of failures, sometimes it can work
> > > 50 times in a row but fail on the 51th).
> > > 
> > > But unfortunately, I forgot to revert this "de-PIEsation" change in
> > > debian stable, and that's exactly where the original bug report come
> > > from, stating kernel broke builds in qemu.
> > > 
> > > The same qemu-user-static configuration has been used by some other
> > > distributions too, but hopefully everything's fixed now.  Except of
> > > debian bookworm, and probably also ubuntu jammy (previous LTS).
> > > 
> > > It is not an "older qemu" anymore (though for a very old qemu this is
> > > true again, that old one can't be used anymore with modern executables
> > > anyway due to other reasons).  It is just my build mistake which is
> > > *still* unfixed on debian stable (bookworm).  And even there, this
> > > issue can trivially be fixed locally, since qemu-user-static is
> > > self-contained and can be installed on older debian releases, and I
> > > always provide up-to-date backports of qemu packages for debian stable.
> > > 
> > > And yes, qemu had numerous improvements in this area since bookworm
> > > version, which addressed many other issues around this and fixed many
> > > other configurations (which are not related to this kernel change),
> > > but the fundamental issue (lack of full-blown MMU) remains.
> > > 
> > > Hopefully this clears things up, and it can be seen that this is not
> > > a kernel bug.  And I'm hoping we'll fix this in debian bookworm soon
> > > too.
> > > 
> > > Thanks, and sorry for all the buzz which caused my 2 mistakes.
> > 
> > So catching up with that as we currently did cherry-pick the revert in
> > Debian but I defintivelfy would like to align with upstream (and drop
> > the cherry-pick again if it's not going to be picked for 6.1.y
> > upstream):
> > 
> > I'm a bit lost here. What are we going to do? Is the commit still
> > temporarly be applied to the stable series or are we staying at the
> > status quo and we should solely deal it within Debian on qemu side to
> > address the issue above and then we are fine? 
> 
> I read this as "oops, we messed up in qemu and will fix it there" and so
> I dropped the reverts from the kernel.  If that's not the case, please
> let me know.

Thanks for the quick confirmation.

Michael, I will drop the local revert in src:linux then on the next
upload I do. Can you handle the qemu part in Debian accordingly (and
make sure the fix lands in the next Debian point release in January
and ideally maybe via stable-updates earlier to affected people?)

Regards,
Salvatore