Commit-ID: cefa929c034eb5d9c15c50088235a0093a219687 Gitweb: https://git.kernel.org/tip/cefa929c034eb5d9c15c50088235a0093a219687 Author: Andy Lutomirski <luto@xxxxxxxxxx> AuthorDate: Thu, 25 Apr 2019 17:11:23 -0700 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Tue, 30 Apr 2019 12:37:50 +0200 x86/mm: Introduce temporary mm structs Using a dedicated page-table for temporary PTEs prevents other cores from using - even speculatively - these PTEs, thereby providing two benefits: (1) Security hardening: an attacker that gains kernel memory writing abilities cannot easily overwrite sensitive data. (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 mm struct, which will follow, is for poking the kernel text. [ Commit message was written by Nadav Amit ] Tested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx> Cc: <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: <ard.biesheuvel@xxxxxxxxxx> Cc: <deneen.t.dock@xxxxxxxxx> Cc: <kernel-hardening@xxxxxxxxxxxxxxxxxx> Cc: <kristen@xxxxxxxxxxxxxxx> Cc: <linux_dti@xxxxxxxxxx> Cc: <will.deacon@xxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20190426001143.4983-4-namit@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- arch/x86/include/asm/mmu_context.h | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index 19d18fae6ec6..24dc3b810970 100644 --- a/arch/x86/include/asm/mmu_context.h +++ b/arch/x86/include/asm/mmu_context.h @@ -356,4 +356,37 @@ static inline unsigned long __get_current_cr3_fast(void) return cr3; } +typedef struct { + struct mm_struct *mm; +} temp_mm_state_t; + +/* + * Using a temporary mm allows to set temporary mappings that are not accessible + * by other CPUs. 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 CPUs. Using a temporary mm also allows to avoid TLB shootdowns when the + * mapping is torn down. + * + * 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 overriding + * the kernel memory protection. + */ +static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm) +{ + temp_mm_state_t temp_state; + + lockdep_assert_irqs_disabled(); + temp_state.mm = this_cpu_read(cpu_tlbstate.loaded_mm); + switch_mm_irqs_off(NULL, mm, current); + return temp_state; +} + +static inline void unuse_temporary_mm(temp_mm_state_t prev_state) +{ + lockdep_assert_irqs_disabled(); + switch_mm_irqs_off(NULL, prev_state.mm, current); +} + #endif /* _ASM_X86_MMU_CONTEXT_H */
