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

From: Borislav Petkov
Date: Tue Dec 08 2020 - 12:51:05 EST

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@xxxxxxxxx>
> ---
> 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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