[PATCH v4 22/23] tlb: provide default nmi_uaccess_okay()

[Rick Edgecombe]

From: Nadav Amit <namit@xxxxxxxxxx>

x86 has an nmi_uaccess_okay(), but other architectures do not.
Arch-independent code might need to know whether access to user
addresses is ok in an NMI context or in other code whose execution
context is unknown.  Specifically, this function is needed for
bpf_probe_write_user().

Add a default implementation of nmi_uaccess_okay() for architectures
that do not have such a function.

Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
 arch/x86/include/asm/tlbflush.h | 2 ++
 include/asm-generic/tlb.h       | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 90926e8dd1f8..dee375831962 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -274,6 +274,8 @@ static inline bool nmi_uaccess_okay(void)
 	return true;
 }
 
+#define nmi_uaccess_okay nmi_uaccess_okay
+
 /* Initialize cr4 shadow for this CPU. */
 static inline void cr4_init_shadow(void)
 {
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index b9edc7608d90..480e5b2a5748 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -21,6 +21,15 @@
 #include <asm/tlbflush.h>
 #include <asm/cacheflush.h>
 
+/*
+ * Blindly accessing user memory from NMI context can be dangerous
+ * if we're in the middle of switching the current user task or switching
+ * the loaded mm.
+ */
+#ifndef nmi_uaccess_okay
+# define nmi_uaccess_okay() true
+#endif
+
 #ifdef CONFIG_MMU
 
 /*
-- 
2.17.1