Re: Avoiding stack buffer clear being optimised out

[Jonny Grant <jg@jguk.org>]

On 30/11/2022 17:40, Jonathan Wakely wrote:
> On Wed, 30 Nov 2022 at 16:27, Jonny Grant <jg@jguk.org> wrote:
>>
>> Hello
>>
>> Does GCC have a clear way to avoid memset being compiled out by optimiser?
>>
>> This article came up, so I combined the broken.c with GCC
>> gcc -Wall -O2 -o broken broken.c
>>
>> Note, I've been using gcc for many years, I'm not looking for just tips how to compile code. I only want to discuss this optimiser issue :-)
>>
>> https://blog.cloudflare.com/the-linux-kernel-key-retention-service-and-why-you-should-use-it-in-your-next-application/
>>
>> If I modify to clear the buffer, it gets removed by the compiler
>>
>> The only way I could get it to not remove the memset is by adding another printf, (propagating a return code after checking memset wasn't enough)
> 
> This is simpler and works for me, but I'm not sure if it's guaranteed
> to always work:
> 
> __attribute__((noinline,noipa))
> void wipe(void* p, size_t n)
> {
>   memset(p, 0, n);
> }
> 
> static int encrypt(void)
> {
>     uint8_t key[] = "hunter2";
>     printf("encrypting with super secret key: %s\n", key);
>     wipe(key, 8);
> }
> 
> There is discussion of alternatives in
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1358.pdf (starting
> on page 6).
> 
> The memset_s function was added to C in Annex K, but most
> implementations of the C library do not support Annex K.


Jonathan

I wonder if you know how GCC decides to remove the memset? In the following example, the memset even changes the bytes on the stack, which are then not changed later on in the program, rather strange.

If I modify the code to check the buffer contains key[0] as nul byte, it still shows as 0, but then the stack is still readable

// gcc -Wall -O2 -o broken broken.c

#include <stdio.h>
#include <stdint.h>
#include <string.h>

static int encrypt(void)
{
    uint8_t key[] = "hunter2";
    printf("encrypting with super secret key: %s\n", key);
    memset(key, 0, 8);
    if(key[0] == '\0') return 0;
    else return 1;
}

static void log_completion(void)
{
    /* oh no, we forgot to init the msg */
    char msg[8];
    printf("not important, just fyi: %s\n", msg);
}

int main(void)
{
    int ret = encrypt();
    /* notify that we're done */
    log_completion();
    printf("ret: %d\n", ret);
    return ret;
}