Messages in this thread | | | Subject | Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp | From | Dave Hansen <> | Date | Mon, 5 Nov 2018 09:20:19 -0800 |
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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
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.
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