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, greg k-h