[PATCH 1/3] New string function explicit_bzero (from OpenBSD).

[Paul Eggert]

On 09/15/2016 06:05 AM, Zack Weinberg wrote:
> +void
> +internal_function
> +__glibc_read_memory(const void *s, size_t len)
> +{
> +  asm volatile ("");
> +}

This leaks the address S to possibly-buggy code, bypassing address 
randomization. For example, if a function body ends in 'explicit_bzero 
(&x, sizeof x)', then &x is likely to be in a machine register or on the 
stack when explicit_bzero returns, more so than if explicit_bzero were 
never called. If a key is stored next to other somewhat-sensitive data 
this will make the somewhat-sensitive data more vulnerable than it 
otherwise would be.

To help avoid this problem, __glibc_read_member could clear all 
caller-save registers (including the return register of course). This 
would not entirely prevent the problem, but that's OK as explicit_bzero 
does not pretend to entirely prevent information leakage.