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SubjectRe: [PATCH RFC 10/24] userfaultfd: wp: add WP pagetable tracking to x86
On Mon, Jan 21, 2019 at 03:57:08PM +0800, Peter Xu wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> Accurate userfaultfd WP tracking is possible by tracking exactly which
> virtual memory ranges were writeprotected by userland. We can't relay
> only on the RW bit of the mapped pagetable because that information is
> destroyed by fork() or KSM or swap. If we were to relay on that, we'd
> need to stay on the safe side and generate false positive wp faults
> for every swapped out page.

So you want to forward write fault (of a protected range) to user space
only if page is not write protected because of fork(), KSM or swap.

This write protection feature is only for anonymous page right ? Other-
wise how would you protect a share page (ie anyone can look it up and
call page_mkwrite on it and start writting to it) ?

So for anonymous page for fork() the mapcount will tell you if page is
write protected for COW. For KSM it is easy check the page flag.

For swap you can use the page lock to synchronize. A page that is
write protected because of swap is write protected because it is being
write to disk thus either under page lock, or with PageWriteback()
returning true while write is on going.

So to me it seems you could properly identify if a page is write
protected for fork, swap or KSM without a new flag.

Cheers,
Jérôme

>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/pgtable.h | 52 ++++++++++++++++++++++++++++
> arch/x86/include/asm/pgtable_64.h | 8 ++++-
> arch/x86/include/asm/pgtable_types.h | 9 +++++
> include/asm-generic/pgtable.h | 1 +
> include/asm-generic/pgtable_uffd.h | 51 +++++++++++++++++++++++++++
> init/Kconfig | 5 +++
> 7 files changed, 126 insertions(+), 1 deletion(-)
> create mode 100644 include/asm-generic/pgtable_uffd.h
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 8689e794a43c..096c773452d0 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -207,6 +207,7 @@ config X86
> select USER_STACKTRACE_SUPPORT
> select VIRT_TO_BUS
> select X86_FEATURE_NAMES if PROC_FS
> + select HAVE_ARCH_USERFAULTFD_WP if USERFAULTFD
>
> config INSTRUCTION_DECODER
> def_bool y
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 40616e805292..7a71158982f4 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -23,6 +23,7 @@
>
> #ifndef __ASSEMBLY__
> #include <asm/x86_init.h>
> +#include <asm-generic/pgtable_uffd.h>
>
> extern pgd_t early_top_pgt[PTRS_PER_PGD];
> int __init __early_make_pgtable(unsigned long address, pmdval_t pmd);
> @@ -293,6 +294,23 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
> return native_make_pte(v & ~clear);
> }
>
> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +static inline int pte_uffd_wp(pte_t pte)
> +{
> + return pte_flags(pte) & _PAGE_UFFD_WP;
> +}
> +
> +static inline pte_t pte_mkuffd_wp(pte_t pte)
> +{
> + return pte_set_flags(pte, _PAGE_UFFD_WP);
> +}
> +
> +static inline pte_t pte_clear_uffd_wp(pte_t pte)
> +{
> + return pte_clear_flags(pte, _PAGE_UFFD_WP);
> +}
> +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
> +
> static inline pte_t pte_mkclean(pte_t pte)
> {
> return pte_clear_flags(pte, _PAGE_DIRTY);
> @@ -372,6 +390,23 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear)
> return native_make_pmd(v & ~clear);
> }
>
> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +static inline int pmd_uffd_wp(pmd_t pmd)
> +{
> + return pmd_flags(pmd) & _PAGE_UFFD_WP;
> +}
> +
> +static inline pmd_t pmd_mkuffd_wp(pmd_t pmd)
> +{
> + return pmd_set_flags(pmd, _PAGE_UFFD_WP);
> +}
> +
> +static inline pmd_t pmd_clear_uffd_wp(pmd_t pmd)
> +{
> + return pmd_clear_flags(pmd, _PAGE_UFFD_WP);
> +}
> +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
> +
> static inline pmd_t pmd_mkold(pmd_t pmd)
> {
> return pmd_clear_flags(pmd, _PAGE_ACCESSED);
> @@ -1351,6 +1386,23 @@ static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t pmd)
> #endif
> #endif
>
> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +static inline pte_t pte_swp_mkuffd_wp(pte_t pte)
> +{
> + return pte_set_flags(pte, _PAGE_SWP_UFFD_WP);
> +}
> +
> +static inline int pte_swp_uffd_wp(pte_t pte)
> +{
> + return pte_flags(pte) & _PAGE_SWP_UFFD_WP;
> +}
> +
> +static inline pte_t pte_swp_clear_uffd_wp(pte_t pte)
> +{
> + return pte_clear_flags(pte, _PAGE_SWP_UFFD_WP);
> +}
> +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
> +
> #define PKRU_AD_BIT 0x1
> #define PKRU_WD_BIT 0x2
> #define PKRU_BITS_PER_PKEY 2
> diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
> index 9c85b54bf03c..e0c5d29b8685 100644
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -189,7 +189,7 @@ extern void sync_global_pgds(unsigned long start, unsigned long end);
> *
> * | ... | 11| 10| 9|8|7|6|5| 4| 3|2| 1|0| <- bit number
> * | ... |SW3|SW2|SW1|G|L|D|A|CD|WT|U| W|P| <- bit names
> - * | TYPE (59-63) | ~OFFSET (9-58) |0|0|X|X| X| X|X|SD|0| <- swp entry
> + * | TYPE (59-63) | ~OFFSET (9-58) |0|0|X|X| X| X|F|SD|0| <- swp entry
> *
> * G (8) is aliased and used as a PROT_NONE indicator for
> * !present ptes. We need to start storing swap entries above
> @@ -197,9 +197,15 @@ extern void sync_global_pgds(unsigned long start, unsigned long end);
> * erratum where they can be incorrectly set by hardware on
> * non-present PTEs.
> *
> + * SD Bits 1-4 are not used in non-present format and available for
> + * special use described below:
> + *
> * SD (1) in swp entry is used to store soft dirty bit, which helps us
> * remember soft dirty over page migration
> *
> + * F (2) in swp entry is used to record when a pagetable is
> + * writeprotected by userfaultfd WP support.
> + *
> * Bit 7 in swp entry should be 0 because pmd_present checks not only P,
> * but also L and G.
> *
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 106b7d0e2dae..163043ab142d 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -32,6 +32,7 @@
>
> #define _PAGE_BIT_SPECIAL _PAGE_BIT_SOFTW1
> #define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW1
> +#define _PAGE_BIT_UFFD_WP _PAGE_BIT_SOFTW2 /* userfaultfd wrprotected */
> #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
> #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
>
> @@ -100,6 +101,14 @@
> #define _PAGE_SWP_SOFT_DIRTY (_AT(pteval_t, 0))
> #endif
>
> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +#define _PAGE_UFFD_WP (_AT(pteval_t, 1) << _PAGE_BIT_UFFD_WP)
> +#define _PAGE_SWP_UFFD_WP _PAGE_USER
> +#else
> +#define _PAGE_UFFD_WP (_AT(pteval_t, 0))
> +#define _PAGE_SWP_UFFD_WP (_AT(pteval_t, 0))
> +#endif
> +
> #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
> #define _PAGE_NX (_AT(pteval_t, 1) << _PAGE_BIT_NX)
> #define _PAGE_DEVMAP (_AT(u64, 1) << _PAGE_BIT_DEVMAP)
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 359fb935ded6..0e1470ecf7b5 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -10,6 +10,7 @@
> #include <linux/mm_types.h>
> #include <linux/bug.h>
> #include <linux/errno.h>
> +#include <asm-generic/pgtable_uffd.h>
>
> #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
> defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
> diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgtable_uffd.h
> new file mode 100644
> index 000000000000..643d1bf559c2
> --- /dev/null
> +++ b/include/asm-generic/pgtable_uffd.h
> @@ -0,0 +1,51 @@
> +#ifndef _ASM_GENERIC_PGTABLE_UFFD_H
> +#define _ASM_GENERIC_PGTABLE_UFFD_H
> +
> +#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> +static __always_inline int pte_uffd_wp(pte_t pte)
> +{
> + return 0;
> +}
> +
> +static __always_inline int pmd_uffd_wp(pmd_t pmd)
> +{
> + return 0;
> +}
> +
> +static __always_inline pte_t pte_mkuffd_wp(pte_t pte)
> +{
> + return pte;
> +}
> +
> +static __always_inline pmd_t pmd_mkuffd_wp(pmd_t pmd)
> +{
> + return pmd;
> +}
> +
> +static __always_inline pte_t pte_clear_uffd_wp(pte_t pte)
> +{
> + return pte;
> +}
> +
> +static __always_inline pmd_t pmd_clear_uffd_wp(pmd_t pmd)
> +{
> + return pmd;
> +}
> +
> +static __always_inline pte_t pte_swp_mkuffd_wp(pte_t pte)
> +{
> + return pte;
> +}
> +
> +static __always_inline int pte_swp_uffd_wp(pte_t pte)
> +{
> + return 0;
> +}
> +
> +static __always_inline pte_t pte_swp_clear_uffd_wp(pte_t pte)
> +{
> + return pte;
> +}
> +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
> +
> +#endif /* _ASM_GENERIC_PGTABLE_UFFD_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index cf5b5a0dcbc2..2a02e004874e 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1418,6 +1418,11 @@ config ADVISE_SYSCALLS
> applications use these syscalls, you can disable this option to save
> space.
>
> +config HAVE_ARCH_USERFAULTFD_WP
> + bool
> + help
> + Arch has userfaultfd write protection support
> +
> config MEMBARRIER
> bool "Enable membarrier() system call" if EXPERT
> default y
> --
> 2.17.1
>

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