Re: [PATCH v4 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Borislav Petkov]

Just textual improvements:

On Fri, Dec 02, 2022 at 04:35:39PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> When Shadow Stack is in use, Write=0,Dirty=1 PTE are reserved for shadow

Pls, no caps.

> stack. Copy-on-write PTes then have Write=0,Cow=1.

"... are preserved for shadow stack pages."

> 
> When a PTE goes from Write=1,Dirty=1 to Write=0,Cow=1, it could
> become a transient shadow stack PTE in two cases:

1. Some processors ...

2. When _PAGE_DIRTY ...

> The first case is that some processors can start a write but end up seeing
> a Write=0 PTE by the time they get to the Dirty bit, creating a transient
> shadow stack PTE. However, this will not occur on processors supporting
> Shadow Stack, and a TLB flush is not necessary.
> 
> The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non-
> atomically, a transient shadow stack PTE can be created as a result.
> Thus, prevent that with cmpxchg.
> 
> In the case of pmdp_set_wrprotect(), for nopmd configs the ->pmd operated
> on does not exist and the logic would need to be different. Although the
> extra functionality will normally be optimized out when user shadow
> stacks are not configured, also exclude it in the preprocessor stage so
> that it will still compile. User shadow stack is not supported there by
> Linux anyway. Leave the cpu_feature_enabled() check so that the
> functionality also disables based on runtime detection of the feature.

"... also gets disabled ..."

Thx.

-- 
Regards/Gruss,
    Boris.

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