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 <torva...@linux-foundation.org>
Suggested-by: Andi Kleen <a...@linux.intel.com>
Cc: Tom Lendacky <thomas.lenda...@amd.com>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Kees Cook <keesc...@chromium.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: x...@kernel.org
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
 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);           \

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