linux-kernel - [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

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Message-Id: <1424199698-7607-1-git-send-email-xypron.glpk@gmx.de>
Date:	Tue, 17 Feb 2015 20:01:38 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
	Heinrich Schuchardt <xypron.glpk@....de>
Subject: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
THREAD_SIZE.

E.g. architecture hexagon may have page size 1M and thread size 4096.

This would lead to a division by zero.

The futex implementation assumes that tids fit into the FUTEX_TID_MASK.
This limits the number of allowable threads.

version 2:
  * use div64_u64
  * check against FUTEX_TID_MASK

Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
---
 kernel/fork.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index cf65139..1449923 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -74,6 +74,7 @@
 #include <linux/uprobes.h>
 #include <linux/aio.h>
 #include <linux/compiler.h>
+#include <linux/math64.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -255,6 +256,8 @@ void __init __weak arch_task_cache_init(void) { }
 
 void __init fork_init(unsigned long mempages)
 {
+	u64 temp;
+
 #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
 #ifndef ARCH_MIN_TASKALIGN
 #define ARCH_MIN_TASKALIGN	L1_CACHE_BYTES
@@ -273,7 +276,16 @@ void __init fork_init(unsigned long mempages)
 	 * value: the thread structures can take up at most half
 	 * of memory.
 	 */
-	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
+	temp = div64_u64((u64) mempages * (u64) PAGE_SIZE,
+			 (u64) THREAD_SIZE * 8UL);
+
+	/*
+	 * The futex code assumes that tids fit into the FUTEX_TID_MASK.
+	 */
+	if (temp < FUTEX_TID_MASK)
+		max_threads = temp;
+	else
+		max_threads = FUTEX_TID_MASK;
 
 	/*
 	 * we need to allow at least 20 threads to boot a system
-- 
2.1.4

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