linux-kernel - [PATCH 3.16 69/76] nospec: Kill array_index_nospec_mask

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Message-ID: <lsq.1520823972.229464947@decadent.org.uk>
Date:   Mon, 12 Mar 2018 03:06:12 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Josh Poimboeuf" <jpoimboe@...hat.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>, linux-arch@...r.kernel.org,
        "David Woodhouse" <dwmw2@...radead.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Ingo Molnar" <mingo@...nel.org>, "Borislav Petkov" <bp@...en8.de>,
        "Arjan van de Ven" <arjan@...ux.intel.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Will Deacon" <will.deacon@....com>,
        "Andy Lutomirski" <luto@...nel.org>
Subject: [PATCH 3.16 69/76] nospec: Kill array_index_nospec_mask_check()

3.16.56-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Dan Williams <dan.j.williams@...el.com>

commit 1d91c1d2c80cb70e2e553845e278b87a960c04da upstream.

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().

Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will.deacon@....com>
Cc: linux-arch@...r.kernel.org
Link: http://lkml.kernel.org/r/151881604278.17395.6605847763178076520.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/nospec.h | 22 +---------------------
 1 file changed, 1 insertion(+), 21 deletions(-)

--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -30,26 +30,6 @@ static inline unsigned long array_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_
 ({									\
 	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));			\