On Thu, Mar 9, 2023 at 6:03 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: > > On Thu, Mar 9, 2023 at 5:13 PM Edgecombe, Rick P > <rick.p.edgecombe@xxxxxxxxx> wrote: > > > > +Joao regarding mixed mode designs > > > > On Fri, 2023-03-10 at 00:51 +0100, Borislav Petkov wrote: > > > On Thu, Mar 09, 2023 at 04:56:37PM +0000, Edgecombe, Rick P wrote: > > > > There is a proc that shows if shadow stack is enabled in a thread. > > > > It > > > > does indeed come later in the series. > > > > > > Not good enough: > > > > > > 1. buried somewhere in proc where no one knows about it > > > > > > 2. it is per thread so user needs to grep *all* > > > > See "x86: Expose thread features in /proc/$PID/status" for the patch. > > We could emit something in dmesg I guess? The logic would be: > > - Record the presence of elf SHSTK bit on exec > > - On shadow stack disable, if it had the elf bit, pr_info("bad!") > > > > > > > > > ... We previously tried to add some batch operations to improve > > > > the > > > > performance, but tglx had suggested to start with something > > > > simple. > > > > So we end up with this simple composable API. > > > > > > I agree with starting simple and thanks for explaining this in > > > detail. > > > > > > TBH, though, it already sounds like a mess to me. I guess a mess > > > we'll > > > have to deal with because there will always be this case of some > > > shared object/lib not being enabled for shstk because of raisins. > > > > The compatibility problems are totally the mess in this whole thing. > > When you try to look at a "permissive" mode that actually works it gets > > even more complex. Joao and I have been banging our heads on that > > problem for months. > > > > But there are some expected users of this that say: we compile and > > check our known set of binaries, we won't get any surprises. So it's > > more of a distro problem. > > > > > > > > And TBH #2, I would've done it even simpler: if some shared object > > > can't > > > do shadow stack, we disable it for the whole process. I mean, what's > > > the > > > point? > > > > You mean a late loaded dlopen()ed DSO? The enabling logic can't know > > this will happen ahead of time. > > > > If you mean if the shared objects in the elf all support shadow stack, > > then this is what happens. The complication is that the loader wants to > > enable shadow stack before it has checked the elf libs so it doesn't > > underflow the shadow stack when it returns from the function that does > > this checking. > > > > So it does: > > 1. Enable shadow stack > > 2. Call elf libs checking functions > > 3. If all good, lock shadow stack. Else, disable shadow stack. > > 4. Return from elf checking functions and if shstk is enabled, don't > > underflow because it was enabled in step 1 and we have return addresses > > from 2 on the shadow stack > > > > I'm wondering if this can't be improved in glibc to look like: > > 1. Check elf libs, and record it somewhere > > 2. Wait until just the right spot > > 3. If all good, enable and lock shadow stack. > > I will try it out. > Currently glibc enables shadow stack as early as possible. There are only a few places where a function call in glibc never returns. We can enable shadow stack just before calling main. There are quite some code paths without shadow stack protection. Is this an issue? H.J.