linux-kernel - [PATCH 1/3] nospec: Kill array_index_nospec_mask

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Date:   Fri, 16 Feb 2018 13:20:42 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     mingo@...nel.org
Cc:     linux-arch@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] nospec: Kill array_index_nospec_mask_check()

There are multiple problems with the dynamic sanity checking in
array_index_nospec_mask_check():

* It causes unnecessary overhead in the 32-bit case since integer sized
  @index values will no longer cause the check to be compiled away like
  in the 64-bit case.

* In the 32-bit case it may trigger with user controllable input when
  the expectation is that should only trigger during development of new
  kernel enabling.

* The macro reuses the input parameter in multiple locations which is
  broken if someone passes an expression like 'index++' to
  array_index_nospec().

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Will Deacon <will.deacon@....com>
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 include/linux/nospec.h |   22 +---------------------
 1 file changed, 1 insertion(+), 21 deletions(-)

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index fbc98e2c8228..d6701e34424f 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -30,26 +30,6 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 #endif
 
 /*
- * Warn developers about inappropriate array_index_nospec() usage.
- *
- * Even if the CPU speculates past the WARN_ONCE branch, the
- * sign bit of @index is taken into account when generating the
- * mask.
- *
- * This warning is compiled out when the compiler can infer that
- * @index and @size are less than LONG_MAX.
- */
-#define array_index_mask_nospec_check(index, size)				\
-({										\
-	if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,			\
-	    "array_index_nospec() limited to range of [0, LONG_MAX]\n"))	\
-		_mask = 0;							\
-	else									\
-		_mask = array_index_mask_nospec(index, size);			\
-	_mask;									\
-})
-
-/*
  * array_index_nospec - sanitize an array index after a bounds check
  *
  * For a code sequence like:
@@ -67,7 +47,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 ({									\
 	typeof(index) _i = (index);					\
 	typeof(size) _s = (size);					\
-	unsigned long _mask = array_index_mask_nospec_check(_i, _s);	\
+	unsigned long _mask = array_index_mask_nospec(_i, _s);		\
 									\
 	BUILD_BUG_ON(sizeof(_i) > sizeof(long));			\
 	BUILD_BUG_ON(sizeof(_s) > sizeof(long));			\