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Date:   Wed, 16 Jan 2019 16:32:45 -0800
From:   Rick Edgecombe <rick.p.edgecombe@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, linux_dti@...oud.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, akpm@...ux-foundation.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        kristen@...ux.intel.com, deneen.t.dock@...el.com,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Nadav Amit <namit@...are.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH 03/17] x86/mm: temporary mm struct

From: Andy Lutomirski <luto@...nel.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@...omium.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
Tested-by: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Nadav Amit <namit@...are.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.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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