On Mon, 8 Feb 2021 at 20:24, Stefano Stabellini <sstabell...@kernel.org> wrote: > > @Ian, I think this wants to go in 4.15. Without it, Xen may receive an IOMMU > > fault for DMA transaction using granted page. > > > > > Backport: 4.12+ > > > > > > --- > > > > > > Given the severity of the bug, I would like to request this patch to be > > > backported to 4.12 too, even if 4.12 is security-fixes only since Oct > > > 2020. > > > > I would agree that the bug is bad, but it is not clear to me why this would > > be > > warrant for an exception for backporting. Can you outline what's the worse > > that can happen? > > > > Correct me if I am wrong, if one can hit this error, then it should be > > pretty > > reliable. Therefore, anyone wanted to use 4.12 in production should have > > seen > > if the error on there setup by now (4.12 has been out for nearly two years). > > If not, then they are most likely not affected. > > > > Any new users of Xen should use the latest stable rather than starting with > > an > > old version. > > Yes, the bug reproduces reliably but it takes more than a smoke test to > find it. That's why it wasn't found by OSSTest and also our internal > CI-loop at Xilinx.
Ok. So a user should be able to catch it during testing, is that correct? > > Users can be very slow at upgrading, so I am worried that 4.12 might still > be picked by somebody, especially given that it is still security > supported for a while. Don't tell me about upgrading Xen... ;) But I am a bit confused, are you worried about existing users or new users? > > > Other than the seriousness of the bug, I think there is also a fairness > > concern. > > > > So far our rules says there is only security support backport allowed. If we > > start granting exception, then we need a way to prevent abuse of it. To take > > an extreme example, why one couldn't ask backport for 4.2? > > > > That said, I vaguely remember this topic was brought up a few time on > > security@. So maybe it is time to have a new discussion about stable tree. > > I wouldn't consider a backport for a tree that is closed even for > security backports. So in your example, I'd say no to a backport to 4.2 > or 4.10. > > I think there is a valid question for trees that are still open to > security fixes but not general backports. > > For these cases, I would just follow a simple rule of thumb: Aren't those rules already used for stable trees? > - is the submitter willing to provide the backport? > - is the backport low-risk? > - is the underlying bug important? You wrote multiple times that this is serious but it is still not clear what's the worse that can happen... > > If the answer to all is "yes" then I'd go with it. > > > In this case, given that the fix is a one-liner, and obviously correct, I have seen one-liners that introduced XSA in the past ;). Cheers,