Re: [PATCH 1/4] [v2] structleak: disable STRUCTLEAK_BYREF in combination with KASAN_STACK

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 28, 2019 at 02:37:46PM +0200, Arnd Bergmann wrote:
> The combination of KASAN_STACK and GCC_PLUGIN_STRUCTLEAK_BYREF
> leads to much larger kernel stack usage, as seen from the warnings
> about functions that now exceed the 2048 byte limit:
> 
> drivers/media/i2c/tvp5150.c:253:1: error: the frame size of 3936 bytes is larger than 2048 bytes
> drivers/media/tuners/r820t.c:1327:1: error: the frame size of 2816 bytes is larger than 2048 bytes
> drivers/net/wireless/broadcom/brcm80211/brcmsmac/phy/phy_n.c:16552:1: error: the frame size of 3144 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> fs/ocfs2/aops.c:1892:1: error: the frame size of 2088 bytes is larger than 2048 bytes
> fs/ocfs2/dlm/dlmrecovery.c:737:1: error: the frame size of 2088 bytes is larger than 2048 bytes
> fs/ocfs2/namei.c:1677:1: error: the frame size of 2584 bytes is larger than 2048 bytes
> fs/ocfs2/super.c:1186:1: error: the frame size of 2640 bytes is larger than 2048 bytes
> fs/ocfs2/xattr.c:3678:1: error: the frame size of 2176 bytes is larger than 2048 bytes
> net/bluetooth/l2cap_core.c:7056:1: error: the frame size of 2144 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> net/bluetooth/l2cap_core.c: In function 'l2cap_recv_frame':
> net/bridge/br_netlink.c:1505:1: error: the frame size of 2448 bytes is larger than 2048 bytes
> net/ieee802154/nl802154.c:548:1: error: the frame size of 2232 bytes is larger than 2048 bytes
> net/wireless/nl80211.c:1726:1: error: the frame size of 2224 bytes is larger than 2048 bytes
> net/wireless/nl80211.c:2357:1: error: the frame size of 4584 bytes is larger than 2048 bytes
> net/wireless/nl80211.c:5108:1: error: the frame size of 2760 bytes is larger than 2048 bytes
> net/wireless/nl80211.c:6472:1: error: the frame size of 2112 bytes is larger than 2048 bytes
> 
> The structleak plugin was previously disabled for CONFIG_COMPILE_TEST,
> but meant we missed some bugs, so this time we should address them.
> 
> The frame size warnings are distracting, and risking a kernel stack
> overflow is generally not beneficial to performance, so it may be best
> to disallow that particular combination. This can be done by turning
> off either one. I picked the dependency in GCC_PLUGIN_STRUCTLEAK_BYREF
> and GCC_PLUGIN_STRUCTLEAK_BYREF_ALL, as this option is designed to
> make uninitialized stack usage less harmful when enabled on its own,
> but it also prevents KASAN from detecting those cases in which it was
> in fact needed.
> 
> KASAN_STACK is currently implied by KASAN on gcc, but could be made a
> user selectable option if we want to allow combining (non-stack) KASAN
> with GCC_PLUGIN_STRUCTLEAK_BYREF.
> 
> Note that it would be possible to specifically address the files that
> print the warning, but presumably the overall stack usage is still
> significantly higher than in other configurations, so this would not
> address the full problem.
> 
> I could not test this with CONFIG_INIT_STACK_ALL, which may or may not
> suffer from a similar problem.
> 
> Fixes: 81a56f6dcd20 ("gcc-plugins: structleak: Generalize to all variable types")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> [v2] do it for both GCC_PLUGIN_STRUCTLEAK_BYREF and GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
> ---
>  security/Kconfig.hardening | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index a1ffe2eb4d5f..af4c979b38ee 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -61,6 +61,7 @@ choice
>  	config GCC_PLUGIN_STRUCTLEAK_BYREF
>  		bool "zero-init structs passed by reference (strong)"
>  		depends on GCC_PLUGINS
> +		depends on !(KASAN && KASAN_STACK=1)
>  		select GCC_PLUGIN_STRUCTLEAK
>  		help
>  		  Zero-initialize any structures on the stack that may
> @@ -70,9 +71,15 @@ choice
>  		  exposures, like CVE-2017-1000410:
>  		  https://git.kernel.org/linus/06e7e776ca4d3654
>  
> +		  As a side-effect, this keeps a lot of variables on the
> +		  stack that can otherwise be optimized out, so combining
> +		  this with CONFIG_KASAN_STACK can lead to a stack overflow
> +		  and is disallowed.
> +
>  	config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
>  		bool "zero-init anything passed by reference (very strong)"
>  		depends on GCC_PLUGINS
> +		depends on !(KASAN && KASAN_STACK=1)
>  		select GCC_PLUGIN_STRUCTLEAK
>  		help
>  		  Zero-initialize any stack variables that may be passed
> -- 
> 2.20.0
> 

-- 
Kees Cook