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Date: Thu, 25 Apr 2019 17:11:24 -0700
From: Nadav Amit <namit@...are.com>
To: Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>,
	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>, Nadav Amit <nadav.amit@...il.com>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <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>, Rick Edgecombe
	<rick.p.edgecombe@...el.com>, Nadav Amit <namit@...are.com>
Subject: [PATCH v5 04/23] x86/mm: Save debug registers when loading a temporary mm

Prevent user watchpoints from mistakenly firing while the temporary mm
is being used. As the addresses of the temporary mm might overlap those
of the user-process, this is necessary to prevent wrong signals or worse
things from happening.

Cc: Andy Lutomirski <luto@...nel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.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 | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 24dc3b810970..93dff1963337 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -13,6 +13,7 @@
 #include <asm/tlbflush.h>
 #include <asm/paravirt.h>
 #include <asm/mpx.h>
+#include <asm/debugreg.h>
 
 extern atomic64_t last_mm_ctx_id;
 
@@ -380,6 +381,21 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 	lockdep_assert_irqs_disabled();
 	temp_state.mm = this_cpu_read(cpu_tlbstate.loaded_mm);
 	switch_mm_irqs_off(NULL, mm, current);
+
+	/*
+	 * If breakpoints are enabled, disable them while the temporary mm is
+	 * used. Userspace might set up watchpoints on addresses that are used
+	 * in the temporary mm, which would lead to wrong signals being sent or
+	 * crashes.
+	 *
+	 * Note that breakpoints are not disabled selectively, which also causes
+	 * kernel breakpoints (e.g., perf's) to be disabled. This might be
+	 * undesirable, but still seems reasonable as the code that runs in the
+	 * temporary mm should be short.
+	 */
+	if (hw_breakpoint_active())
+		hw_breakpoint_disable();
+
 	return temp_state;
 }
 
@@ -387,6 +403,13 @@ 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);
+
+	/*
+	 * Restore the breakpoints if they were disabled before the temporary mm
+	 * was loaded.
+	 */
+	if (hw_breakpoint_active())
+		hw_breakpoint_restore();
 }
 
 #endif /* _ASM_X86_MMU_CONTEXT_H */
-- 
2.17.1

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