[PATCH 4.14 059/195] [Variant 1/Spectre-v1] arm64: Use pointer masking to limit uaccess speculation

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Robin Murphy <robin.mur...@arm.com>


Commit 4d8efc2d5ee4 upstream.

Similarly to x86, mitigate speculation past an access_ok() check by
masking the pointer against the address limit before use.

Even if we don't expect speculative writes per se, it is plausible that
a CPU may still speculate at least as far as fetching a cache line for
writing, hence we also harden put_user() and clear_user() for peace of
mind.

Signed-off-by: Robin Murphy <robin.mur...@arm.com>
Signed-off-by: Will Deacon <will.dea...@arm.com>
Signed-off-by: Catalin Marinas <catalin.mari...@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 arch/arm64/include/asm/uaccess.h |   26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -216,6 +216,26 @@ static inline void uaccess_enable_not_ua
 }
 
 /*
+ * Sanitise a uaccess pointer such that it becomes NULL if above the
+ * current addr_limit.
+ */
+#define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
+static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
+{
+       void __user *safe_ptr;
+
+       asm volatile(
+       "       bics    xzr, %1, %2\n"
+       "       csel    %0, %1, xzr, eq\n"
+       : "=&r" (safe_ptr)
+       : "r" (ptr), "r" (current_thread_info()->addr_limit)
+       : "cc");
+
+       csdb();
+       return safe_ptr;
+}
+
+/*
  * The "__xxx" versions of the user access functions do not verify the address
  * space - it must have been done previously with a separate "access_ok()"
  * call.
@@ -285,7 +305,7 @@ do {                                                        
                \
        __typeof__(*(ptr)) __user *__p = (ptr);                         \
        might_fault();                                                  \
        access_ok(VERIFY_READ, __p, sizeof(*__p)) ?                     \
-               __get_user((x), __p) :                                  \
+               __p = uaccess_mask_ptr(__p), __get_user((x), __p) :     \
                ((x) = 0, -EFAULT);                                     \
 })
 
@@ -349,7 +369,7 @@ do {                                                        
                \
        __typeof__(*(ptr)) __user *__p = (ptr);                         \
        might_fault();                                                  \
        access_ok(VERIFY_WRITE, __p, sizeof(*__p)) ?                    \
-               __put_user((x), __p) :                                  \
+               __p = uaccess_mask_ptr(__p), __put_user((x), __p) :     \
                -EFAULT;                                                \
 })
 
@@ -365,7 +385,7 @@ extern unsigned long __must_check __clea
 static inline unsigned long __must_check clear_user(void __user *to, unsigned 
long n)
 {
        if (access_ok(VERIFY_WRITE, to, n))
-               n = __clear_user(to, n);
+               n = __clear_user(__uaccess_mask_ptr(to), n);
        return n;
 }