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Date:   Wed, 16 Nov 2022 11:09:43 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
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Subject: Re: [PATCH v3 15/37] x86/mm: Check Shadow Stack page fault errors
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On Tue, Nov 15, 2022 at 11:13:34PM +0000, Edgecombe, Rick P wrote:

> When a page becomes COW it changes from a shadow stack permissioned
> page (Write=0,Dirty=1) to (Write=0,Dirty=0,CoW=1), which is simply
> read-only to the CPU. When shadow stack is enabled, a RET would
> normally pop the shadow stack by reading it with a "shadow stack read"
> access. However, in the COW case the shadow stack memory does not have
> shadow stack permissions, it is read-only. So it will generate a fault.
> 
> For conventionally writable pages, a read can be serviced with a read
> only PTE, and COW would not have to happen. But for shadow stack, there
> isn't the concept of read-only shadow stack memory. If it is shadow
> stack permissioned, it can be modified via CALL and RET instructions.
> So COW needs to happen before any memory can be mapped with shadow
> stack permissions.
> 
> Shadow stack accesses (read or write) need to be serviced with shadow
> stack permissioned memory, so in the case of a shadow stack read
> access, treat it as a WRITE fault so both COW will happen and the write
> fault path will tickle maybe_mkwrite() and map the memory shadow stack.

ACK.