Re: [PATCH v20 08/25] x86/mm: Introduce _PAGE_COW

From: Yu, Yu-cheng
Date: Mon Feb 15 2021 - 12:35:42 EST

On 2/10/2021 12:28 PM, Yu, Yu-cheng wrote:
On 2/10/2021 11:42 AM, Kees Cook wrote:
On Wed, Feb 10, 2021 at 09:56:46AM -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.

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.

In places where Linux normally creates Write=0, Dirty=1, it can use the
software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY.  In other
words, whenever Linux needs to create Write=0, Dirty=1, it instead creates
Write=0, Cow=1, except for shadow stack, which is Write=0, Dirty=1. This
clearly separates shadow stack from other data, and results in the
following:

(a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
(b) A R/O page that has been COW'ed: (Write=0, Cow=1)
     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 Write=0 and Cow=1.
(c) A shadow stack PTE: (Write=0, Dirty=1)
(d) A shared shadow stack PTE: (Write=0, Cow=1)
     When a shadow stack page is being shared among processes (this happens
     at fork()), its PTE is made Dirty=0, so the next shadow stack access
     causes a fault, and the page is duplicated and Dirty=1 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.  That's possible today, but
     will not happen on processors that support shadow stack.

Define _PAGE_COW and update pte_*() helpers and apply the same changes to
pmd and pud.

I still find this commit confusing mostly due to _PAGE_COW being 0
without CET enabled. Shouldn't this just get changed universally? Why
should this change depend on CET?


For example, in...

static inline int pte_write(pte_t pte)
{
    if (cpu_feature_enabled(X86_FEATURE_SHSTK))
        return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
    else
        return pte_flags(pte) & _PAGE_RW;
}

There are four cases:

(a) RW=1, Dirty=1 -> writable
(b) RW=1, Dirty=0 -> writable
(c) RW=0, Dirty=0 -> not writable
(d) RW=0, Dirty=1 -> shadow stack, or not-writable if !X86_FEATURE_SHSTK

Case (d) is ture only when shadow stack is enabled, otherwise it is not writable.  With shadow stack feature, the usual dirty, copy-on-write PTE becomes RW=0, Cow=1.

We can get this changed universally, but all usual dirty, copy-on-write PTEs need the Dirty/Cow swapping, always.  Is that desirable?


Instead of working on _PAGE_COW directly, create three helpers: pte_make_cow(), pte_clear_cow(), pte_shstk(). The decision of swapping _PAGE_DIRTY/_PAGE_COW is now in the helpers. For example, pte_wrprotect() is now:

static inline pte_t pte_wrprotect(pte_t pte)
{
pte = pte_clear_flags(pte, _PAGE_RW);

if (pte_dirty(pte))
pte = pte_make_cow(pte);
return pte;
}

How is that? The revise patch is the following.

Yu-cheng




diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index a02c67291cfc..1a6c0784af0a 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -121,11 +121,21 @@ 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;
+ /*
+ * A dirty PTE has Dirty=1 or Cow=1.
+ */
+ return pte_flags(pte) & _PAGE_DIRTY_BITS;
}

+static inline bool pte_shstk(pte_t pte)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return false;
+
+ return (pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
+}

static inline u32 read_pkru(void)
{
@@ -160,9 +170,20 @@ static inline int pte_young(pte_t pte)
return pte_flags(pte) & _PAGE_ACCESSED;
}

-static inline int pmd_dirty(pmd_t pmd)
+static inline bool pmd_dirty(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_DIRTY;
+ /*
+ * A dirty PMD has Dirty=1 or Cow=1.
+ */
+ return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
+}
+
+static inline bool pmd_shstk(pmd_t pmd)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return false;
+
+ return (pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
}

static inline int pmd_young(pmd_t pmd)
@@ -170,9 +191,12 @@ static inline int pmd_young(pmd_t pmd)
return pmd_flags(pmd) & _PAGE_ACCESSED;
}

-static inline int pud_dirty(pud_t pud)
+static inline bool pud_dirty(pud_t pud)
{
- return pud_flags(pud) & _PAGE_DIRTY;
+ /*
+ * A dirty PUD has Dirty=1 or Cow=1.
+ */
+ return pud_flags(pud) & _PAGE_DIRTY_BITS;
}

static inline int pud_young(pud_t pud)
@@ -182,7 +206,7 @@ static inline int pud_young(pud_t pud)

static inline int pte_write(pte_t pte)
{
- return pte_flags(pte) & _PAGE_RW;
+ return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
}

static inline int pte_huge(pte_t pte)
@@ -314,6 +338,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
return native_make_pte(v & ~clear);
}

+static inline pte_t pte_make_cow(pte_t pte)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pte;
+
+ pte = pte_clear_flags(pte, _PAGE_DIRTY);
+ return pte_set_flags(pte, _PAGE_COW);
+}
+
+static inline pte_t pte_clear_cow(pte_t pte)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pte;
+
+ pte = pte_set_flags(pte, _PAGE_DIRTY);
+ return pte_clear_flags(pte, _PAGE_COW);
+}
+
#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
static inline int pte_uffd_wp(pte_t pte)
{
@@ -333,7 +375,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)

static inline pte_t pte_mkclean(pte_t pte)
{
- return pte_clear_flags(pte, _PAGE_DIRTY);
+ return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
}

static inline pte_t pte_mkold(pte_t pte)
@@ -343,7 +385,16 @@ static inline pte_t pte_mkold(pte_t pte)

static inline pte_t pte_wrprotect(pte_t pte)
{
- return pte_clear_flags(pte, _PAGE_RW);
+ pte = pte_clear_flags(pte, _PAGE_RW);
+
+ /*
+ * 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 (pte_dirty(pte))
+ pte = pte_make_cow(pte);
+ return pte;
}

static inline pte_t pte_mkexec(pte_t pte)
@@ -353,7 +404,18 @@ static inline pte_t pte_mkexec(pte_t pte)

static inline pte_t pte_mkdirty(pte_t pte)
{
- return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+ pteval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PTEs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !pte_write(pte))
+ dirty = _PAGE_COW;
+
+ return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
+}
+
+static inline pte_t pte_mkwrite_shstk(pte_t pte)
+{
+ return pte_clear_cow(pte);
}

static inline pte_t pte_mkyoung(pte_t pte)
@@ -363,7 +425,12 @@ static inline pte_t pte_mkyoung(pte_t pte)

static inline pte_t pte_mkwrite(pte_t pte)
{
- return pte_set_flags(pte, _PAGE_RW);
+ pte = pte_set_flags(pte, _PAGE_RW);
+
+ if (pte_dirty(pte))
+ pte = pte_clear_cow(pte);
+
+ return pte;
}

static inline pte_t pte_mkhuge(pte_t pte)
@@ -410,6 +477,24 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
return native_make_pmd(v & ~clear);
}

+static inline pmd_t pmd_make_cow(pmd_t pmd)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pmd;
+
+ pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
+ return pmd_set_flags(pmd, _PAGE_COW);
+}
+
+static inline pmd_t pmd_clear_cow(pmd_t pmd)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pmd;
+
+ pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
+ return pmd_clear_flags(pmd, _PAGE_COW);
+}
+
#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
static inline int pmd_uffd_wp(pmd_t pmd)
{
@@ -434,17 +519,36 @@ static inline pmd_t pmd_mkold(pmd_t pmd)

static inline pmd_t pmd_mkclean(pmd_t pmd)
{
- return pmd_clear_flags(pmd, _PAGE_DIRTY);
+ return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
}

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

static inline pmd_t pmd_mkdirty(pmd_t pmd)
{
- return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+ pmdval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PMDs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))
+ dirty = _PAGE_COW;
+
+ return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
+}
+
+static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
+{
+ return pmd_clear_cow(pmd);
}

static inline pmd_t pmd_mkdevmap(pmd_t pmd)
@@ -464,7 +568,11 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)

static inline pmd_t pmd_mkwrite(pmd_t pmd)
{
- return pmd_set_flags(pmd, _PAGE_RW);
+ pmd = pmd_set_flags(pmd, _PAGE_RW);
+
+ if (pmd_dirty(pmd))
+ pmd = pmd_clear_cow(pmd);
+ return pmd;
}

static inline pud_t pud_set_flags(pud_t pud, pudval_t set)
@@ -481,6 +589,24 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear)
return native_make_pud(v & ~clear);
}

+static inline pud_t pud_make_cow(pud_t pud)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pud;
+
+ pud = pud_clear_flags(pud, _PAGE_DIRTY);
+ return pud_set_flags(pud, _PAGE_COW);
+}
+
+static inline pud_t pud_clear_cow(pud_t pud)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return pud;
+
+ pud = pud_set_flags(pud, _PAGE_DIRTY);
+ return pud_clear_flags(pud, _PAGE_COW);
+}
+
static inline pud_t pud_mkold(pud_t pud)
{
return pud_clear_flags(pud, _PAGE_ACCESSED);
@@ -488,17 +614,32 @@ static inline pud_t pud_mkold(pud_t pud)

static inline pud_t pud_mkclean(pud_t pud)
{
- return pud_clear_flags(pud, _PAGE_DIRTY);
+ return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
}

static inline pud_t pud_wrprotect(pud_t pud)
{
- return pud_clear_flags(pud, _PAGE_RW);
+ pud = pud_clear_flags(pud, _PAGE_RW);
+
+ /*
+ * Blindly clearing _PAGE_RW might accidentally create
+ * a shadow stack PUD (RW=0, Dirty=1). Move the hardware
+ * dirty value to the software bit.
+ */
+ if (pud_dirty(pud))
+ pud = pud_make_cow(pud);
+ return pud;
}

static inline pud_t pud_mkdirty(pud_t pud)
{
- return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+ pudval_t dirty = _PAGE_DIRTY;
+
+ /* Avoid creating (HW)Dirty=1, Write=0 PUDs */
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))
+ dirty = _PAGE_COW;
+
+ return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
}

static inline pud_t pud_mkdevmap(pud_t pud)
@@ -518,7 +659,11 @@ static inline pud_t pud_mkyoung(pud_t pud)

static inline pud_t pud_mkwrite(pud_t pud)
{
- return pud_set_flags(pud, _PAGE_RW);
+ pud = pud_set_flags(pud, _PAGE_RW);
+
+ if (pud_dirty(pud))
+ pud = pud_clear_cow(pud);
+ return pud;
}

#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
@@ -1131,7 +1276,7 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
#define pmd_write pmd_write
static inline int pmd_write(pmd_t pmd)
{
- return pmd_flags(pmd) & _PAGE_RW;
+ return (pmd_flags(pmd) & _PAGE_RW) || pmd_shstk(pmd);
}

#define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR