On Tue, May 31, 2022 at 10:34 AM Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Tue, 2022-05-31 at 19:36 +0300, Mike Rapoport wrote: > > > WRSS is a feature where you would usually want to lock it as > > > disabled, > > > but WRSS cannot be enabled if shadow stack is not enabled. Locking > > > shadow stack and WRSS off together doesn't have any security > > > benefits > > > in theory. so I'm thinking glibc doesn't need to do this. The > > > kernel > > > could even refuse to lock WRSS without shadow stack being enabled. > > > Could we avoid the extra ptrace functionality then? > > > > What I see for is that a program can support shadow stack, glibc > > enables > > shadow stack, does not enable WRSS and than calls > > > > arch_prctl(ARCH_X86_FEATURE_LOCK, > > LINUX_X86_FEATURE_SHSTK | LINUX_X86_FEATURE_WRSS); > > I see the logic is glibc will lock SHSTK|IBT if either is enabled in > the elf header. I guess that is why I didn't see the locking happening > for me, because my manual enablement test doesn't have either set in > the header. > > It can't see where that glibc knows about WRSS though... > > The glibc logic seems wrong to me also, because shadow stack or IBT > could be force-disabled via glibc tunables. I don't see why the elf > header bit should exclusively control the feature locking. Or why both > should be locked if only one is in the header. glibc locks SHSTK and IBT only if they are enabled at run-time. It doesn't enable/disable/lock WRSS at the moment. If WRSS can be enabled via arch_prctl at any time, we can't lock it. If WRSS should be locked early, how should it be enabled in application? Also can WRSS be enabled from a dlopened object? > > > > so that WRSS cannot be re-enabled. > > > > For the programs that do not support shadow stack, both SHSTK and > > WRSS are > > disabled, but still there is the same call to > > arch_prctl(ARCH_X86_FEATURE_LOCK, ...) and then neither shadow stack > > nor > > WRSS can be enabled. > > > > My original plan was to run CRIU with no shadow stack, enable shadow > > stack > > and WRSS in the restored tasks using arch_prct() and after the shadow > > stack > > contents is restored disable WRSS. > > > > Obviously, this didn't work with glibc I have :) > > Were you disabling shadow stack via glibc tunnable? Or was the elf > header marked for IBT? If it was a plain old binary, the code looks to > me like it should not lock any features. > > > > > On the bright side, having a ptrace call to unlock shadow stack and > > wrss > > allows running CRIU itself with shadow stack. > > > > Yea, having something working is really great. My only hesitancy is > that, per a discussion on the LAM patchset, we are going to make this > enabling API CET only (same semantics for though). I suppose the > locking API arch_prctl() could still be support other arch features, > but it might be a second CET only regset. It's not the end of the > world. > > I guess the other consideration is tieing CRIU to glibc peculiarities. > Like even if we fix glibc, then CRIU may not work with some other libc > or app that force disables for some weird reason. Is it supposed to be > libc-agnostic? > -- H.J.
