For 'get_user' paths, do not allow the kernel to speculate on the value
of a user controlled pointer. In addition to the 'stac' instruction for
Supervisor Mode Access Protection, an 'ifence' causes the 'access_ok'
result to resolve in the pipeline before the cpu might take any
speculative action on the pointer value.
Since this is a major kernel interface that deals with user controlled
data, the '__uaccess_begin_nospec' mechanism will prevent speculative
execution past an 'access_ok' permission check. While speculative
execution past 'access_ok' is not enough to lead to a kernel memory
leak, it is a necessary precondition.
To be clear, '__uaccess_begin_nospec' and ASM_IFENCE are not addressing
any known issues with 'get_user' they are addressing a class of
potential problems that could be near 'get_user' usages. In other words,
these helpers are for hygiene not clinical fixes.
There are no functional changes in this patch.
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Suggested-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
arch/x86/include/asm/smap.h | 4 ++++
arch/x86/include/asm/uaccess.h | 10 ++++++++++
2 files changed, 14 insertions(+)
diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
index db333300bd4b..0b59707e0b46 100644
--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -40,6 +40,10 @@
#endif /* CONFIG_X86_SMAP */
+#define ASM_IFENCE \
+ ALTERNATIVE_2 "", "mfence", X86_FEATURE_MFENCE_RDTSC, \
+ "lfence", X86_FEATURE_LFENCE_RDTSC
+
#else /* __ASSEMBLY__ */
#include <asm/alternative.h>
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 574dff4d2913..a31fd4fc6483 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -124,6 +124,11 @@ extern int __get_user_bad(void);
#define __uaccess_begin() stac()
#define __uaccess_end() clac()
+#define __uaccess_begin_nospec() \
+({ \
+ stac(); \
+ ifence(); \
+})
/*
* This is a type: either unsigned long, if the argument fits into
@@ -487,6 +492,11 @@ struct __large_struct { unsigned long buf[100]; };
__uaccess_begin(); \
barrier();
+#define uaccess_try_nospec do { \
+ current->thread.uaccess_err = 0; \
+ __uaccess_begin_nospec(); \
+ barrier();
+
#define uaccess_catch(err) \
__uaccess_end(); \
(err) |= (current->thread.uaccess_err ? -EFAULT : 0); \
