On Mon, Nov 5, 2018 at 9:20 AM Dave Hansen <[email protected]> wrote: > > > On 11/4/18 9:14 PM, Andy Lutomirski wrote: > > I should add: if this patch is *not* applied, then I think we'll > > need to replace the sw_error_code check with user_mode(regs) to avoid > > an info leak if CET is enabled. Because, with CET, WRUSS will allow > > a *kernel* mode access (where regs->sp is the kernel stack pointer) > > with user permissions. > > Are you saying that WRUSS, if it faults will set the "user" page fault > error code bit? I seem to have some rough recollection about it being > that way, and the shadow-stack spec does say: > > paging access control checks will be treated as a user-mode > shadow stack store >
I believe so, and it would make sense for it to work this way. I would love some instructions for directly accessing normal user memory, too. Maybe a prefix? > But the SDM says: > > For all instruction fetches and most data accesses, this > distinction is determined by the current privilege level (CPL): > accesses made while CPL < 3 are supervisor-mode accesses, while > accesses made while CPL = 3 are user-mode accesses. > > It would certainly be ideal if things affecting the core architecture > like this were in the SDM itself before we merged them. It makes things > like this a lot easier to figure out. Agreed. The current documentation situation is not so good.
