Re: [PATCH v7 01/11] Revert "[PATCH] uml: export symbols added by GCC hardened"

Nick Desaulniers
2023-06-09T21:24:30Z

On Thu, Jun 8, 2023 at 7:24 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> This reverts commit cead61a6717a9873426b08d73a34a325e3546f5d.
>
> It exported __stack_smash_handler and __guard, while they may not be
> defined by anyone.
>
> The code *declares* __stack_smash_handler and __guard. It does not
> create weak symbols. When the stack-protector is disabled, they are
> left undefined, but yet exported.
>
> If a loadable module tries to access non-existing symbols, bad things
> (a page fault, NULL pointer dereference, etc.) will happen. So, the
> current code is wrong.
>
> If the code were written as follows, it would *define* them as weak
> symbols so modules would be able to get access to them.
>
> void (*__stack_smash_handler)(void *) __attribute__((weak));
> EXPORT_SYMBOL(__stack_smash_handler);
>
> long __guard __attribute__((weak));
> EXPORT_SYMBOL(__guard);
>
> In fact, modpost forbids exporting undefined symbols. It shows an error
> message if it detects such a mistake.
>
> ERROR: modpost: "..." [...] was exported without definition
>
> Unfortunately, it is checked only when the code is built as modular.
> The problem described above has been unnoticed for a long time because
> arch/um/os-Linux/user_syms.c is always built-in.
>
> With a planned change in Kbuild, exporting undefined symbols will always
> result in a build error instead of a run-time error. It is a good thing,
> but we need to fix the breakage in advance.
>
> One fix is to *define* weak symbols as shown above. An alternative is
> to export them conditionally as follows:
>
> #ifdef CONFIG_STACKPROTECTOR
> extern void __stack_smash_handler(void *);
> EXPORT_SYMBOL(__stack_smash_handler);
>
> external long __guard;
> EXPORT_SYMBOL(__guard);
> #endif
>
> This is what other architectures do; EXPORT_SYMBOL(__stack_chk_guard)
> is guarded by #ifdef CONFIG_STACKPROTECTOR.
>
> However, adding the #ifdef guard is not sensible because UML cannot
> enable the stack-protector in the first place! (Please note UML does
> not select HAVE_STACKPROTECTOR in Kconfig.)
>
> So, the code is already broken (and unused) in multiple ways.
>
> Just remove.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>
> Changes in v7:
> - New patch
>
> arch/um/os-Linux/user_syms.c | 7 -------
> 1 file changed, 7 deletions(-)
>
> diff --git a/arch/um/os-Linux/user_syms.c b/arch/um/os-Linux/user_syms.c
> index 9b62a9d352b3..a310ae27b479 100644
> --- a/arch/um/os-Linux/user_syms.c
> +++ b/arch/um/os-Linux/user_syms.c
> @@ -37,13 +37,6 @@ EXPORT_SYMBOL(vsyscall_ehdr);
> EXPORT_SYMBOL(vsyscall_end);
> #endif
>
> -/* Export symbols used by GCC for the stack protector. */
> -extern void __stack_smash_handler(void *) __attribute__((weak));
> -EXPORT_SYMBOL(__stack_smash_handler);
> -
> -extern long __guard __attribute__((weak));
> -EXPORT_SYMBOL(__guard);
> -
> #ifdef _FORTIFY_SOURCE
> extern int __sprintf_chk(char *str, int flag, size_t len, const char *format);
> EXPORT_SYMBOL(__sprintf_chk);
> --
> 2.39.2
>

--
Thanks,
~Nick Desaulniers