Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

[Kees Cook]

On Thu, Apr 23, 2020 at 06:12:24PM +0200, Borislav Petkov wrote:
> Ok,
> 
> I have tried to summarize our odyssey so far and here's what I came up
> with. Just built latest gcc from the git repo and it seems to work.
> 
> Next I need to come up with a slick way of testing the compiler...
> 
> Thx.
> 
> ---
> From: Borislav Petkov <bp@xxxxxxx>
> 
> ... or the odyssey of trying to disable the stack protector for the
> function which generates the stack canary value.
> 
> The whole story started with Sergei reporting a boot crash with a kernel
> built with gcc-10:
> 
>   Kernel panic â not syncing: stack-protector: Kernel stack is corrupted in: start_secondary
>   CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc5â00235âgfffb08b37df9 #139
>   Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77MâD3H, BIOS F12 11/14/2013
>   Call Trace:
>     dump_stack
>     panic
>     ? start_secondary
>     __stack_chk_fail
>     start_secondary
>     secondary_startup_64
>   -â-[ end Kernel panic â not syncing: stackâprotector: Kernel stack is corrupted in: start_secondary
> 
> This happens because gcc-10 tail-call optimizes the last function call
> in start_secondary() - cpu_startup_entry() - and thus emits a stack
> canary check which fails because the canary value changes after the
> boot_init_stack_canary() call.
> 
> To fix that, the initial attempt was to mark the one function which
> generates the stack canary with:
> 
>   __attribute__((optimize("-fno-stack-protector"))) ... start_secondary(void *unused)
> 
> however, using the optimize attribute doesn't work cumulatively
> as the attribute does not add to but rather replaces previously
> supplied optimization options - roughly all -fxxx options.
> 
> The key one among them being -fno-omit-frame-pointer and thus leading to
> not present frame pointer - frame pointer which the kernel needs.
> 
> The next attempt to prevent compilers from tail-call optimizing
> the last function call cpu_startup_entry(), shy of carving out
> start_secondary() into a separate compilation unit and building it with
> -fno-stack-protector, is this one.
> 
> The current solution is short and sweet, and reportedly, is supported by
> both compilers so let's see how far we'll get this time.
> 
> Reported-by: Sergei Trofimovich <slyfox@xxxxxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

I'm glad to have the gcc bug opened for the function attribute; thanks
Nick! I needed that for the syscall entry code, but instead went with
a compilation-unit down-grade[1]. I'd much prefer the attribute. :)

-Kees

[1] https://lore.kernel.org/lkml/20200406231606.37619-5-keescook@xxxxxxxxxxxx/

> Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@xxxxxxxxxx
> ---
>  arch/x86/kernel/smpboot.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 3b9bf8c7e29d..e9f44727fccd 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -266,6 +266,14 @@ static void notrace start_secondary(void *unused)
>  
>  	wmb();
>  	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> +
> +	/*
> +	 * Prevent tail call to cpu_startup_entry() because the stack protector
> +	 * guard has been changed a couple of functions up, in
> +	 * boot_init_stack_canary() and must not be checked before tail calling
> +	 * another function.
> +	 */
> +	asm ("");
>  }
>  
>  /**
> -- 
> 2.21.0
> 
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

-- 
Kees Cook