Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW

[Borislav Petkov <bp@alien8.de>]

On Tue, Nov 10, 2020 at 08:21:53AM -0800, Yu-cheng Yu wrote:
> There is essentially no room left in the x86 hardware PTEs on some OSes
> (not Linux).  That left the hardware architects looking for a way to
> represent a new memory type (shadow stack) within the existing bits.
> They chose to repurpose a lightly-used state: Write=0,Dirty=1.

It is not clear to me what the definition and semantics of that bit is.

+#define _PAGE_BIT_COW          _PAGE_BIT_SOFTW5 /* copy-on-write */

Is it set by hw or by sw and hw uses it to know it is a shadow stack
page, and so on.

I think you should lead with its definition.

> The reason it's lightly used is that Dirty=1 is normally set by hardware
> and cannot normally be set by hardware on a Write=0 PTE.  Software must
> normally be involved to create one of these PTEs, so software can simply
> opt to not create them.
> 
> But that leaves us with a Linux problem: we need to ensure we never create

Please use passive voice in your commit message: no "we" or "I", etc.

> Write=0,Dirty=1 PTEs.  In places where we do create them, we need to find
> an alternative way to represent them _without_ using the same hardware bit
> combination.  Thus, enter _PAGE_COW.  This results in the following:
> 
> (a) A modified, copy-on-write (COW) page: (R/O + _PAGE_COW)
> (b) A R/O page that has been COW'ed: (R/O + _PAGE_COW)

Both are "R/O + _PAGE_COW". Where's the difference? The dirty bit?

>     The user page is in a R/O VMA, and get_user_pages() needs a writable
>     copy.  The page fault handler creates a copy of the page and sets
>     the new copy's PTE as R/O and _PAGE_COW.
> (c) A shadow stack PTE: (R/O + _PAGE_DIRTY_HW)

So W=0, D=1 ?

> (d) A shared shadow stack PTE: (R/O + _PAGE_COW)
>     When a shadow stack page is being shared among processes (this happens
>     at fork()), its PTE is cleared of _PAGE_DIRTY_HW, so the next shadow
>     stack access causes a fault, and the page is duplicated and
>     _PAGE_DIRTY_HW is set again.  This is the COW equivalent for shadow
>     stack pages, even though it's copy-on-access rather than copy-on-write.
> (e) A page where the processor observed a Write=1 PTE, started a write, set
>     Dirty=1, but then observed a Write=0 PTE.

How does that happen? Something changed the PTE's W bit to 0 in-between?

> That's possible today, but
>     will not happen on processors that support shadow stack.
> 
> Use _PAGE_COW in pte_wrprotect() and _PAGE_DIRTY_HW in pte_mkwrite().
> Apply the same changes to pmd and pud.
> 
> When this patch is applied, there are six free bits left in the 64-bit PTE.

s/When this patch is applied/After this/

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> There are no more free bits in the 32-bit PTE (except for PAE) and shadow
> stack is not implemented for the 32-bit kernel.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
>  arch/x86/include/asm/pgtable.h       | 120 ++++++++++++++++++++++++---
>  arch/x86/include/asm/pgtable_types.h |  41 ++++++++-
>  2 files changed, 150 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index b23697658b28..c88c7ccf0318 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -121,9 +121,9 @@ extern pmdval_t early_pmd_flags;
>   * The following only work if pte_present() is true.
>   * Undefined behaviour if not..
>   */
> -static inline int pte_dirty(pte_t pte)
> +static inline bool pte_dirty(pte_t pte)
>  {
> -	return pte_flags(pte) & _PAGE_DIRTY_HW;
> +	return pte_flags(pte) & _PAGE_DIRTY_BITS;

Why?

Does _PAGE_COW mean dirty too?

> @@ -343,6 +349,17 @@ static inline pte_t pte_mkold(pte_t pte)
>  
>  static inline pte_t pte_wrprotect(pte_t pte)
>  {
> +	/*
> +	 * Blindly clearing _PAGE_RW might accidentally create
> +	 * a shadow stack PTE (RW=0,Dirty=1).  Move the hardware
> +	 * dirty value to the software bit.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		pte.pte |= (pte.pte & _PAGE_DIRTY_HW) >>
> +			   _PAGE_BIT_DIRTY_HW << _PAGE_BIT_COW;

Let that line stick out. And that shifting is not grokkable at a quick
glance, at least not to me. Simplify?

>  static inline pmd_t pmd_wrprotect(pmd_t pmd)
>  {
> +	/*
> +	 * Blindly clearing _PAGE_RW might accidentally create
> +	 * a shadow stack PMD (RW=0,Dirty=1).  Move the hardware
> +	 * dirty value to the software bit.

This whole carefully sidestepping the possiblity of creating a shadow
stack pXd is kinda sucky...

> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 7462a574fc93..5f764d8d9bae 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -23,7 +23,8 @@
>  #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 */
> @@ -36,6 +37,16 @@
>  #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_SOFTW3 /* software dirty tracking */
>  #define _PAGE_BIT_DEVMAP	_PAGE_BIT_SOFTW4
>  
> +/*
> + * This bit indicates a copy-on-write page, and is different from
> + * _PAGE_BIT_SOFT_DIRTY, which tracks which pages a task writes to.
> + */
> +#ifdef CONFIG_X86_64

CONFIG_X86_64 ? Do all x86 machines out there support CET?

If anything, CONFIG_X86_CET...

> +#define _PAGE_BIT_COW		_PAGE_BIT_SOFTW5 /* copy-on-write */
> +#else
> +#define _PAGE_BIT_COW		0
> +#endif
> +
>  /* If _PAGE_BIT_PRESENT is clear, we use these: */
>  /* - if the user mapped it with PROT_NONE; pte_present gives true */
-- 
Regards/Gruss,
    Boris.

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