On Mon, Jul 22, 2019 at 1:41 PM Arnd Bergmann <[email protected]> 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.
We would love to be able to run KASAN together with
CONFIG_INIT_STACK_ALL on syzbot, as this will potentially reduce the
number of flaky errors.
Given that we already increase the stack size in KASAN builds, how big
of a problem are these warnings?
Perhaps it's better to disable them in this configuration, or push the limit up?
> Fixes: 81a56f6dcd20 ("gcc-plugins: structleak: Generalize to all variable types")
> Acked-by: Kees Cook <[email protected]>
> Link: https://lore.kernel.org/lkml/[email protected]/
> Signed-off-by: Arnd Bergmann <[email protected]>
> ---
> [v2] do it for both GCC_PLUGIN_STRUCTLEAK_BYREF and GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
>
> Andrew, can you pick this up in -mm? It looks like nobody else
> wanted it in their trees even though there was agreement on the
> patch itself.
> ---
> 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
>
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
On Mon, Jul 22, 2019 at 3:43 PM Alexander Potapenko <[email protected]> wrote:
> On Mon, Jul 22, 2019 at 1:41 PM Arnd Bergmann <[email protected]> wrote:
> >
> > 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.
> We would love to be able to run KASAN together with
> CONFIG_INIT_STACK_ALL on syzbot, as this will potentially reduce the
> number of flaky errors.
Doesn't that just limit the usefulness of KASAN, as you no longer catch
actual accesses to unintialized variables that KASAN is designed to find?
> Given that we already increase the stack size in KASAN builds, how big
> of a problem are these warnings?
> Perhaps it's better to disable them in this configuration, or push the limit up?
I'm really hoping to lower the per-function limit for 'allmodconfig' builds,
since both a high limit and lots of bogus warnings prevent us from
noticing any newly introduced functions that use a lot of kernel stack
without KASAN.
An allmodconfig build (and ideally also any randconfig build) should always
complete without warnings to be useful for compile testing.
Arnd