On Fri, Dec 02, 2022 at 04:35:32PM -0800, Rick Edgecombe wrote: > From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > > Shadow stack register state can be managed with XSAVE. The registers > can logically be separated into two groups: > * Registers controlling user-mode operation > * Registers controlling kernel-mode operation > > The architecture has two new XSAVE state components: one for each group > of those groups of registers. This lets an OS manage them separately if > it chooses. Future patches for host userspace and KVM guests will only > utilize the user-mode registers, so only configure XSAVE to save > user-mode registers. This state will add 16 bytes to the xsave buffer > size. > > Future patches will use the user-mode XSAVE area to save guest user-mode > CET state. However, VMCS includes new fields for guest CET supervisor > states. KVM can use these to save and restore guest supervisor state, so > host supervisor XSAVE support is not required. > > Adding this exacerbates the already unwieldy if statement in > check_xstate_against_struct() that handles warning about un-implemented > xfeatures. So refactor these check's by having XCHECK_SZ() set a bool when > it actually check's the xfeature. This ends up exceeding 80 chars, but was > better on balance than other options explored. Pass the bool as pointer to > make it clear that XCHECK_SZ() can change the variable. > > While configuring user-mode XSAVE, clarify kernel-mode registers are not > managed by XSAVE by defining the xfeature in > XFEATURE_MASK_SUPERVISOR_UNSUPPORTED, like is done for XFEATURE_MASK_PT. > This serves more of a documentation as code purpose, and functionally, > only enables a few safety checks. > > Both XSAVE state components are supervisor states, even the state > controlling user-mode operation. This is a departure from earlier features > like protection keys where the PKRU state is a normal user > (non-supervisor) state. Having the user state be supervisor-managed > ensures there is no direct, unprivileged access to it, making it harder > for an attacker to subvert CET. > > To facilitate this privileged access, define the two user-mode CET MSRs, > and the bits defined in those MSRs relevant to future shadow stack > enablement patches. > > Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx> > Tested-by: John Allen <john.allen@xxxxxxx> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -- Kees Cook
