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Date: Thu, 6 Jul 2017 13:09:02 +0200
From: Arnd Bergmann <arnd@...db.de>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, Kees Cook <keescook@...omium.org>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/RFT PATCH] gcc-plugins: force initialize auto variables
whose addresses are taken
On Thu, Jul 6, 2017 at 12:13 PM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> To prevent leaking stack contents in cases where it is not possible
> for the compiler to figure out whether an automatic variable has been
> initialized or not, add a plugin that forcibly initializes all automatic
> variables of struct/union types if their address is taken at any point.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
If we only do this for variables that have their address taken, we miss
a lot of cases that the compiler (both clang and gcc) decide not to warn
about but that can still cause undefined behavior, e.g.:
extern int g(void);
int f(void)
{
int i;
switch (g()) {
case 1:
i = 0;
}
return i;
}
which gets compiled without warning under the assumption that
g() always returns '1':
0000000000000000 <f>:
0: 48 83 ec 08 sub $0x8,%rsp
4: e8 00 00 00 00 callq 9 <f+0x9>
5: R_X86_64_PC32 g-0x4
9: b8 00 00 00 00 mov $0x0,%eax
e: 48 83 c4 08 add $0x8,%rsp
12: c3 retq
Detecting those cases from the plugin may be a lot harder.
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -443,6 +443,15 @@ config GCC_PLUGIN_STRUCTLEAK_VERBOSE
> initialized. Since not all existing initializers are detected
> by the plugin, this can produce false positive warnings.
>
> +config GCC_PLUGIN_INITAUTOBYREF
> + bool "Force initialization of auto variables that have their address taken"
> + depends on GCC_PLUGINS
> + help
> +
> +config GCC_PLUGIN_INITAUTOBYREF_VERBOSE
> + bool "Report uninitialized auto variables that have their address taken"
> + depends on GCC_PLUGIN_INITAUTOBYREF
I think this should be
depends on GCC_PLUGIN_INITAUTOBYREF || COMPILE_TEST
to avoid producing output in an allmodconfig build.
Arnd
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