On Mon, Nov 7, 2022 at 1:11 PM Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Mon, 2022-11-07 at 11:10 -0800, H.J. Lu wrote: > > On Mon, Nov 7, 2022 at 9:47 AM Edgecombe, Rick P > > <rick.p.edgecombe@xxxxxxxxx> wrote: > > > > > > On Mon, 2022-11-07 at 17:55 +0100, Florian Weimer wrote: > > > > * Rick P. Edgecombe: > > > > > > > > > On Sun, 2022-11-06 at 10:33 +0100, Florian Weimer wrote: > > > > > > * H. J. Lu: > > > > > > > > > > > > > This change doesn't make a binary CET compatible. It just > > > > > > > requires > > > > > > > that the toolchain must be updated and all binaries have to > > > > > > > be > > > > > > > recompiled with the new toolchain to enable CET. It > > > > > > > doesn't > > > > > > > solve > > > > > > > any > > > > > > > issue which can't be solved by not updating glibc. > > > > > > > > > > > > Right, and it doesn't even address the library case (the > > > > > > kernel > > > > > > would > > > > > > have to hook into mmap for that). The kernel shouldn't do > > > > > > this. > > > > > > > > > > Shadow stack shouldn't enable as a result of loading a library, > > > > > if > > > > > that's what you mean. > > > > > > > > It's the opposite—loading incompatible libraries needs to disable > > > > shadow > > > > stack (or ideally, not enable it in the first place). > > > > > > The glibc changes I have been using would not have enabled shadow > > > stack > > > in the first place unless the execing binary has the elf bit. So > > > the > > > binary would run as if shadow stack was not enabled in the kernel > > > and > > > there should be nothing to disable when an incompatible binary is > > > loaded. Glibc will have to detect this and act accordingly because > > > not > > > all kernels will have shadow stack configured. > > > > > > > Technically, I > > > > think most incompatible code resides in libraries, so this kernel > > > > change > > > > achieves nothing besides punishing early implementations of the > > > > published-as-finalized x86-64 ABI. > > > > > > It's under the assumption that not breaking things is more > > > important > > > than having shadow stack enabled. So it is not intended as a > > > punishment > > > for users at all, rather the opposite. > > > > > > I'm not sure how much the spec mandates things by the letter of it, > > > but > > > in any case things have gone wrong in the real world. I am very > > > open to > > > discussion here. I only went this way as a last resort because I > > > didn't > > > hear back on the last thread. > > > > Some applications and libraries are compiled with -fcf-protection, > > but > > they manipulate the stack in such a way that they aren't compatible > > with the shadow stack. However, if the build/test setup doesn't > > support > > shadow stack, it is impossible to validate. > > > > When we have everything in place, the problems would be much more > obvious when distros started turning it on. But we can't turn it on as Not necessarily. The problem will show up only in a CET enabled environment since build/test setup may not be on a CET capable hardware. > planned without breaking things for existing binaries. We can have both > by: > 1. Choosing a new bit, adding it to the tools, and never supporting the > old bit in glibc. > 2. Providing the option to have the kernel block the old bit, so > upgraded users can decide what experience they would like. Then distros > can find the problems and adjust their packages. I'm starting to think > a default off sysctl toggle might be better than a Kconfig. > 3. Any other ideas? Don't enable CET in glibc until we can validate CET functionality. -- H.J.
