'ifence_array_ptr' is provided as an alternative to the default
'__array_ptr' implementation that uses a mask to sanitize user
controllable pointers. Later patches will allow it to be selected via
the kernel command line. The '__array_ptr' implementation otherwise
appears safe for current generation cpus across multiple architectures.

'array_ptr_mask' is used by the default 'array_ptr' implementation to
cheaply calculate an array bounds mask.

Suggested-by: Linus Torvalds <torva...@linux-foundation.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
 arch/x86/include/asm/barrier.h |   46 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index b04f572d6d97..1b507f9e2cc7 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -28,6 +28,52 @@
 #define ifence() alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC, \
                                   "lfence", X86_FEATURE_LFENCE_RDTSC)
 
+/**
+ * ifence_array_ptr - Generate a pointer to an array element,
+ * ensuring the pointer is bounded under speculation.
+ *
+ * @arr: the base of the array
+ * @idx: the index of the element
+ * @sz: the number of elements in the array
+ *
+ * If @idx falls in the interval [0, @sz), returns the pointer to
+ * @arr[@idx], otherwise returns NULL.
+ */
+#define ifence_array_ptr(arr, idx, sz)                                 \
+({                                                                     \
+       typeof(*(arr)) *__arr = (arr), *__ret;                          \
+       typeof(idx) __idx = (idx);                                      \
+       typeof(sz) __sz = (sz);                                         \
+                                                                       \
+       __ret = __idx < __sz ? __arr + __idx : NULL;                    \
+       ifence();                                                       \
+       __ret;                                                          \
+})
+
+/**
+ * array_ptr_mask - generate a mask for array_ptr() that is ~0UL when
+ * the bounds check succeeds and 0 otherwise
+ */
+#define array_ptr_mask array_ptr_mask
+static inline unsigned long array_ptr_mask(unsigned long idx, unsigned long sz)
+{
+       unsigned long mask;
+
+       /*
+        * mask = index - size, if that result is >= 0 then the index is
+        * invalid and the mask is 0 else ~0
+        */
+#ifdef CONFIG_X86_32
+       asm ("cmpl %1,%2; sbbl %0,%0;"
+#else
+       asm ("cmpq %1,%2; sbbq %0,%0;"
+#endif
+                       :"=r" (mask)
+                       :"r"(sz),"r" (idx)
+                       :"cc");
+       return mask;
+}
+
 #ifdef CONFIG_X86_PPRO_FENCE
 #define dma_rmb()      rmb()
 #else

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