[RFC PATCH v2 13/15] arm64: mm: Guard page table writes with kpkeys
Qi Zheng
zhengqi.arch at bytedance.com
Wed Jan 8 23:17:33 PST 2025
Hi Kevin,
On 2025/1/8 18:32, Kevin Brodsky wrote:
> When CONFIG_KPKEYS_HARDENED_PGTABLES is enabled, page tables (both
> user and kernel) are mapped with a privileged pkey in the linear
> mapping. As a result, they can only be written under the
> kpkeys_hardened_pgtables guard, which sets POR_EL1 appropriately to
> allow such writes.
>
> Use this guard wherever page tables genuinely need to be written,
> keeping its scope as small as possible (so that POR_EL1 is reset as
> fast as possible). Where atomics are involved, the guard's scope
> encompasses the whole loop to avoid switching POR_EL1 unnecessarily.
>
> This patch is a no-op if CONFIG_KPKEYS_HARDENED_PGTABLES is disabled
> (default).
>
> Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
> ---
> arch/arm64/include/asm/pgtable.h | 19 +++++++++++++++++--
> arch/arm64/mm/fault.c | 2 ++
> 2 files changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index f8dac6673887..0d60a49dc234 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -39,6 +39,7 @@
> #include <linux/mm_types.h>
> #include <linux/sched.h>
> #include <linux/page_table_check.h>
> +#include <linux/kpkeys.h>
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
> @@ -314,6 +315,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>
> static inline void __set_pte_nosync(pte_t *ptep, pte_t pte)
> {
> + guard(kpkeys_hardened_pgtables)();
> WRITE_ONCE(*ptep, pte);
> }
>
> @@ -758,6 +760,7 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
> }
> #endif /* __PAGETABLE_PMD_FOLDED */
>
> + guard(kpkeys_hardened_pgtables)();
> WRITE_ONCE(*pmdp, pmd);
>
> if (pmd_valid(pmd)) {
I noticed a long time ago that set_pte/set_pmd/... was implemented
separately by each architecture without a unified entry point. This
makes it difficult to add some hooks for them.
Taking set_pte() as an example, is it possible to do the following:
1) add a generic set_pte() in include/asm-generic/tlb.h (Or other more
appropriate files)
static inline void set_pte(pte_t *ptep, pte_t pte)
{
arch_set_pte(ptep, pte);
}
2) let each architecture include this file and rename the original
set_pte() to arch_set_pte().
3) then we can add hooks for generic set_pte():
static inline void set_pte(pte_t *ptep, pte_t pte)
{
guard(kpkeys_hardened_pgtables)();
arch_set_pte(ptep, pte);
}
4) in this way, the architecture that supports
ARCH_HAS_KPKEYS_HARDENED_PGTABLES only needs to implement
the kpkeys_hardened_pgtables(), otherwise it is no-op.
Just some immature ideas, and the related set/clear interfaces
are currently quite messy. ;)
Of course, this does not affect the feature to be implemented
in this patch series.
Thanks!
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