[PATCH v5 4/4] kasan: disable freed user page poisoning with HW tags
Andrey Konovalov
andreyknvl at gmail.com
Wed Jun 2 05:20:12 PDT 2021
On Tue, Jun 1, 2021 at 10:51 PM Peter Collingbourne <pcc at google.com> wrote:
>
> Poisoning freed pages protects against kernel use-after-free. The
> likelihood of such a bug involving kernel pages is significantly higher
> than that for user pages. At the same time, poisoning freed pages can
> impose a significant performance cost, which cannot always be justified
> for user pages given the lower probability of finding a bug. Therefore,
> disable freed user page poisoning when using HW tags. We identify
> "user" pages via the flag set GFP_HIGHUSER_MOVABLE, which indicates
> a strong likelihood of not being directly accessible to the kernel.
>
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Link: https://linux-review.googlesource.com/id/I716846e2de8ef179f44e835770df7e6307be96c9
> ---
> v4:
> - move flag to GFP_HIGHUSER_MOVABLE
> - remove command line flag
>
> include/linux/gfp.h | 13 ++++++++++---
> include/linux/page-flags.h | 9 +++++++++
> include/trace/events/mmflags.h | 9 ++++++++-
> mm/kasan/hw_tags.c | 3 +++
> mm/page_alloc.c | 12 +++++++-----
> 5 files changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 68ba237365dc..e6102dfa4faa 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -54,8 +54,9 @@ struct vm_area_struct;
> #define ___GFP_THISNODE 0x200000u
> #define ___GFP_ACCOUNT 0x400000u
> #define ___GFP_ZEROTAGS 0x800000u
> +#define ___GFP_SKIP_KASAN_POISON 0x1000000u
> #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP 0x1000000u
> +#define ___GFP_NOLOCKDEP 0x2000000u
> #else
> #define ___GFP_NOLOCKDEP 0
> #endif
> @@ -233,17 +234,22 @@ struct vm_area_struct;
> *
> * %__GFP_ZEROTAGS returns a page with zeroed memory tags on success, if
> * __GFP_ZERO is set.
> + *
> + * %__GFP_SKIP_KASAN_POISON returns a page which does not need to be poisoned
> + * on deallocation. Typically used for userspace pages. Currently only has an
> + * effect in HW tags mode.
> */
> #define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN)
> #define __GFP_COMP ((__force gfp_t)___GFP_COMP)
> #define __GFP_ZERO ((__force gfp_t)___GFP_ZERO)
> #define __GFP_ZEROTAGS ((__force gfp_t)___GFP_ZEROTAGS)
> +#define __GFP_SKIP_KASAN_POISON ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
>
> /* Disable lockdep for GFP context tracking */
> #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>
> /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (24 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
> #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
> /**
> @@ -324,7 +330,8 @@ struct vm_area_struct;
> #define GFP_DMA __GFP_DMA
> #define GFP_DMA32 __GFP_DMA32
> #define GFP_HIGHUSER (GFP_USER | __GFP_HIGHMEM)
> -#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
> +#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE | \
> + __GFP_SKIP_KASAN_POISON)
> #define GFP_TRANSHUGE_LIGHT ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
> __GFP_NOMEMALLOC | __GFP_NOWARN) & ~__GFP_RECLAIM)
> #define GFP_TRANSHUGE (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 04a34c08e0a6..40e2c5000585 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -137,6 +137,9 @@ enum pageflags {
> #endif
> #ifdef CONFIG_64BIT
> PG_arch_2,
> +#endif
> +#ifdef CONFIG_KASAN_HW_TAGS
> + PG_skip_kasan_poison,
> #endif
> __NR_PAGEFLAGS,
>
> @@ -443,6 +446,12 @@ TESTCLEARFLAG(Young, young, PF_ANY)
> PAGEFLAG(Idle, idle, PF_ANY)
> #endif
>
> +#ifdef CONFIG_KASAN_HW_TAGS
> +PAGEFLAG(SkipKASanPoison, skip_kasan_poison, PF_HEAD)
> +#else
> +PAGEFLAG_FALSE(SkipKASanPoison)
> +#endif
> +
> /*
> * PageReported() is used to track reported free pages within the Buddy
> * allocator. We can use the non-atomic version of the test and set
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 629c7a0eaff2..390270e00a1d 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -85,6 +85,12 @@
> #define IF_HAVE_PG_ARCH_2(flag,string)
> #endif
>
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string) ,{1UL << flag, string}
> +#else
> +#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string)
> +#endif
> +
> #define __def_pageflag_names \
> {1UL << PG_locked, "locked" }, \
> {1UL << PG_waiters, "waiters" }, \
> @@ -112,7 +118,8 @@ IF_HAVE_PG_UNCACHED(PG_uncached, "uncached" ) \
> IF_HAVE_PG_HWPOISON(PG_hwpoison, "hwpoison" ) \
> IF_HAVE_PG_IDLE(PG_young, "young" ) \
> IF_HAVE_PG_IDLE(PG_idle, "idle" ) \
> -IF_HAVE_PG_ARCH_2(PG_arch_2, "arch_2" )
> +IF_HAVE_PG_ARCH_2(PG_arch_2, "arch_2" ) \
> +IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
>
> #define show_page_flags(flags) \
> (flags) ? __print_flags(flags, "|", \
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 41fd5326ee0a..ed5e5b833d61 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -246,6 +246,9 @@ void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags)
> */
> bool init = !want_init_on_free() && want_init_on_alloc(flags);
>
> + if (flags & __GFP_SKIP_KASAN_POISON)
> + SetPageSkipKASanPoison(page);
> +
> if (flags & __GFP_ZEROTAGS) {
> int i;
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 13937e793fda..5ad76e540a22 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -394,11 +394,12 @@ static DEFINE_STATIC_KEY_TRUE(deferred_pages);
> * on-demand allocation and then freed again before the deferred pages
> * initialization is done, but this is not likely to happen.
> */
> -static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
> +static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
> {
> return static_branch_unlikely(&deferred_pages) ||
> (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
> - (fpi_flags & FPI_SKIP_KASAN_POISON));
> + (fpi_flags & FPI_SKIP_KASAN_POISON)) ||
> + PageSkipKASanPoison(page);
> }
>
> /* Returns true if the struct page for the pfn is uninitialised */
> @@ -449,10 +450,11 @@ defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> return false;
> }
> #else
> -static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
> +static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
> {
> return (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
> - (fpi_flags & FPI_SKIP_KASAN_POISON));
> + (fpi_flags & FPI_SKIP_KASAN_POISON)) ||
> + PageSkipKASanPoison(page);
> }
>
> static inline bool early_page_uninitialised(unsigned long pfn)
> @@ -1244,7 +1246,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
> unsigned int order, bool check_free, fpi_t fpi_flags)
> {
> int bad = 0;
> - bool skip_kasan_poison = should_skip_kasan_poison(fpi_flags);
> + bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
>
> VM_BUG_ON_PAGE(PageTail(page), page);
>
> --
> 2.32.0.rc1.229.g3e70b5a671-goog
>
Reviewed-by: Andrey Konovalov <andreyknvl at gmail.com>
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