Messages in this thread Patch in this message | | | From | Nadav Amit <> | Subject | [PATCH v5 03/10] x86/mm: temporary mm struct | Date | Tue, 13 Nov 2018 05:07:23 -0800 |
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From: Andy Lutomirski <luto@kernel.org>
Sometimes we want to set a temporary page-table entries (PTEs) in one of the cores, without allowing other cores to use - even speculatively - these mappings. There are two benefits for doing so:
(1) Security: if sensitive PTEs are set, temporary mm prevents their use in other cores. This hardens the security as it prevents exploding a dangling pointer to overwrite sensitive data using the sensitive PTE.
(2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in remote page-tables.
To do so a temporary mm_struct can be used. Mappings which are private for this mm can be set in the userspace part of the address-space. During the whole time in which the temporary mm is loaded, interrupts must be disabled.
The first use-case for temporary PTEs, which will follow, is for poking the kernel text.
[ Commit message was written by Nadav ]
Cc: Kees Cook <keescook@chromium.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Dave Hansen <dave.hansen@intel.com> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org> Tested-by: Masami Hiramatsu <mhiramat@kernel.org> Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Nadav Amit <namit@vmware.com> --- arch/x86/include/asm/mmu_context.h | 32 ++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+)
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index 0ca50611e8ce..0141b7fa6d01 100644 --- a/arch/x86/include/asm/mmu_context.h +++ b/arch/x86/include/asm/mmu_context.h @@ -338,4 +338,36 @@ static inline unsigned long __get_current_cr3_fast(void) return cr3; } +typedef struct { + struct mm_struct *prev; +} temporary_mm_state_t; + +/* + * Using a temporary mm allows to set temporary mappings that are not accessible + * by other cores. Such mappings are needed to perform sensitive memory writes + * that override the kernel memory protections (e.g., W^X), without exposing the + * temporary page-table mappings that are required for these write operations to + * other cores. + * + * Context: The temporary mm needs to be used exclusively by a single core. To + * harden security IRQs must be disabled while the temporary mm is + * loaded, thereby preventing interrupt handler bugs from override the + * kernel memory protection. + */ +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm) +{ + temporary_mm_state_t state; + + lockdep_assert_irqs_disabled(); + state.prev = this_cpu_read(cpu_tlbstate.loaded_mm); + switch_mm_irqs_off(NULL, mm, current); + return state; +} + +static inline void unuse_temporary_mm(temporary_mm_state_t prev) +{ + lockdep_assert_irqs_disabled(); + switch_mm_irqs_off(NULL, prev.prev, current); +} + #endif /* _ASM_X86_MMU_CONTEXT_H */ -- 2.17.1
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