linux-kernel - Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED

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Date:   Thu, 2 Mar 2023 17:01:25 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "bp@...en8.de" <bp@...en8.de>
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Subject: Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

On Thu, 2023-03-02 at 13:48 +0100, Borislav Petkov wrote:
> On Mon, Feb 27, 2023 at 02:29:30PM -0800, Rick Edgecombe wrote:
> > diff --git a/arch/x86/include/asm/pgtable_types.h
> > b/arch/x86/include/asm/pgtable_types.h
> > index 0646ad00178b..56b374d1bffb 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -21,7 +21,8 @@
> >   #define _PAGE_BIT_SOFTW2     10      /* " */
> >   #define _PAGE_BIT_SOFTW3     11      /* " */
> >   #define _PAGE_BIT_PAT_LARGE  12      /* On 2MB or 1GB pages */
> > -#define _PAGE_BIT_SOFTW4     58      /* available for programmer
> > */
> > +#define _PAGE_BIT_SOFTW4     57      /* available for programmer
> > */
> > +#define _PAGE_BIT_SOFTW5     58      /* available for programmer
> > */
> >   #define _PAGE_BIT_PKEY_BIT0  59      /* Protection Keys, bit 1/4
> > */
> >   #define _PAGE_BIT_PKEY_BIT1  60      /* Protection Keys, bit 2/4
> > */
> >   #define _PAGE_BIT_PKEY_BIT2  61      /* Protection Keys, bit 3/4
> > */
> > @@ -34,6 +35,15 @@
> >   #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty
> > tracking */
> >   #define _PAGE_BIT_DEVMAP     _PAGE_BIT_SOFTW4
> >   
> > +/*
> > + * Indicates a Saved Dirty bit page.
> > + */
> > +#ifdef CONFIG_X86_USER_SHADOW_STACK
> > +#define _PAGE_BIT_SAVED_DIRTY                _PAGE_BIT_SOFTW5 /*
> > Saved Dirty bit */
> > +#else
> > +#define _PAGE_BIT_SAVED_DIRTY                0
> > +#endif
> > +
> >   /* If _PAGE_BIT_PRESENT is clear, we use these: */
> >   /* - if the user mapped it with PROT_NONE; pte_present gives true
> > */
> >   #define _PAGE_BIT_PROTNONE   _PAGE_BIT_GLOBAL
> > @@ -117,6 +127,25 @@
> >   #define _PAGE_SOFTW4 (_AT(pteval_t, 0))
> >   #endif
> >   
> > +/*
> > + * The hardware requires shadow stack to be Write=0,Dirty=1.
> > However,
> > + * there are valid cases where the kernel might create read-only
> > PTEs that
> > + * are dirty (e.g., fork(), mprotect(), uffd-wp(), soft-dirty 
> > tracking). In
> > + * this case, the _PAGE_SAVED_DIRTY bit is used instead of the HW-
> > dirty bit,
> > + * to avoid creating a wrong "shadow stack" PTEs. Such PTEs have
> > + * (Write=0,SavedDirty=1,Dirty=0) set.
> > + *
> > + * Note that on processors without shadow stack support, the 
> 
> .git/rebase-apply/patch:154: trailing whitespace.
>  * Note that on processors without shadow stack support, the 
> warning: 1 line adds whitespace errors.
> 
> Hm, apparently git checks for that too - not only trailing empty
> lines.

Weird. And oops on the space. Just wondering how checkpatch missed
this. It didn't, just was in a pile of false positives on that patch
and I didn't notice it in there.