linux-kernel - Re: Is kernel optimized with dead store removal?

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Date:	Thu, 25 Feb 2010 16:14:01 +0100
From:	Roel Kluin <roel.kluin@...il.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org
Subject: Re: Is kernel optimized with dead store removal?


>  >  Does this optimization also occur during compilation of the Linux
>  >  kernel?

> Any such dead store removal is up to the compiler and the lifetime
> of the object being clobbered. For 'auto' objects the optimization
> is certainly likely.
> 
> This is only a problem if the memory (a thread stack, say) is recycled
> and leaked uninitialized to user-space, but such bugs are squashed
> fairly quickly upon discovery.

Thanks for comments,

In the sha1_update() case I don't know whether the stack is recycled and
leaked - it may be dependent on the calling function, but isn't it
vulnerable?

I tested this with the snippet below. If compiled with -O1 or -O2 and
ON_STACK defined 1, I can read "Secret" a second time. With ON_STACK
defined 0 I do not.

Roel

---
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define ON_STACK 1

void foo()
{
        char password[] = "secret";
        password[0]='S';
        printf ("Don't show again: %s\n", password);
        memset(password, 0, sizeof(password));
}

void foo2()
{
        char* password = malloc(7);
        strncpy (password, "secret" , 7);
        password[6] = '\0';
        password[0] = 'S';
        printf ("Don't show again: %s\n", password);
        memset(password, 0, 7);
        free(password);

}

int main(int argc, char* argv[])
{

#if ON_STACK == 1
        foo();
#else
        foo2();
#endif
        int i;
        char foo3[] = "hoi";
        printf ("foo1:%s\n", foo3);
        char* bar = &foo3[0];
        for (i = -50; i < 50; i++)
                printf ("%c.", bar[i]);
        printf("\n");
        return 0;
}
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