linux-kernel - [PATH] Fix stack randomization on x86

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Date:	Wed, 07 Jan 2015 20:47:54 +0100
From:	Hector Marco Gisbert <hecmargi@....es>
To:	linux-kernel@...r.kernel.org
Cc:	Kees Cook <keescook@...omium.org>,
	Ismael Ripoll Ripoll <iripoll@...ca.upv.es>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: [PATH] Fix stack randomization on x86_64 bit


[PATH] Fix stack randomization on x86_64 bit

The issue is that the stack for processes is not properly randomized on 64 bit
architectures due to an integer overflow.


The affected function is randomize_stack_top() in file "fs/binfmt_elf.c":

static unsigned long randomize_stack_top(unsigned long stack_top)
{
         unsigned int random_variable = 0;

         if ((current->flags & PF_RANDOMIZE) &&
                 !(current->personality & ADDR_NO_RANDOMIZE)) {
                 random_variable = get_random_int() & STACK_RND_MASK;
                 random_variable <<= PAGE_SHIFT;
         }
#ifdef CONFIG_STACK_GROWSUP
         return PAGE_ALIGN(stack_top) + random_variable;
#else
         return PAGE_ALIGN(stack_top) - random_variable;
#endif
}


Note that, it declares the "random_variable" variable as "unsigned int". Since
the result of the shifting operation between STACK_RND_MASK (which is  
0x3fffff on
x86_64, 22 bits) and PAGE_SHIFT (which is 12 on x86_64):

random_variable <<= PAGE_SHIFT;

then the two leftmost bits are dropped when storing the result in the
"random_variable". This variable shall be at least 34 bits long to hold the
(22+12) result.

These two dropped bits have an impact on the entropy of process stack.
Concretely, the total stack entropy is reduced by four: from 2^28 to 2^30 (One
fourth of expected entropy).

This patch restores back the entropy by correcting the types involved in the
operations in the functions randomize_stack_top() and stack_maxrandom_size().


Signed-off-by: Hector Marco-Gisbert <hecmargi@....es>
Signed-off-by: Ismael Ripoll <iripoll@....es>


diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 919b912..df4552b 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -35,12 +35,12 @@ struct va_alignment __read_mostly va_align = {
  	.flags = -1,
  };

-static unsigned int stack_maxrandom_size(void)
+static unsigned long stack_maxrandom_size(void)
  {
-	unsigned int max = 0;
+	unsigned long max = 0;
  	if ((current->flags & PF_RANDOMIZE) &&
  		!(current->personality & ADDR_NO_RANDOMIZE)) {
-		max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
+		max = ((-1UL) & STACK_RND_MASK) << PAGE_SHIFT;
  	}

  	return max;
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index d8fc060..ee668b4 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -554,11 +554,11 @@ out:

  static unsigned long randomize_stack_top(unsigned long stack_top)
  {
-	unsigned int random_variable = 0;
+	unsigned long random_variable = 0;

  	if ((current->flags & PF_RANDOMIZE) &&
  		!(current->personality & ADDR_NO_RANDOMIZE)) {
-		random_variable = get_random_int() & STACK_RND_MASK;
+		random_variable = (unsigned long) get_random_int() & STACK_RND_MASK;
  		random_variable <<= PAGE_SHIFT;
  	}
  #ifdef CONFIG_STACK_GROWSUP


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