[PATCH v1 1/4] syscalls: Restore address limit after a syscall

Kees Cook
2017-03-08T21:57:09Z

On Wed, Mar 8, 2017 at 1:38 PM, Thomas Garnier <thgarnie@google.com> wrote:
> This patch prevents a syscall to modify the address limit of the
> caller. The address limit is kept by the syscall wrapper and restored
> just after the syscall ends.
>
> For example, it would mitigation this bug:
>
> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> By default, this change warns if the segment is incorrect while
> returning to user-mode and fix it. The
> CONFIG_VERIFY_PRE_USERMODE_STATE_BUG option can be enabled to halt
> instead if needed.

Instead of this new config, please reuse the CHECK_DATA_CORRUPTION
test instead, which already controls very similar WARN vs BUG
behavior. Example below...

>
> The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
> added so each architecture can optimize how the
> verify_pre_usermode_state function is called.
>
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
> Based on next-20170308
> ---
> include/linux/syscalls.h | 19 +++++++++++++++++++
> init/Kconfig | 16 ++++++++++++++++
> kernel/sys.c | 11 +++++++++++
> 3 files changed, 46 insertions(+)
>
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 980c3c9b06f8..78a2268ecd6e 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -191,6 +191,22 @@ extern struct trace_event_functions exit_syscall_print_funcs;
> SYSCALL_METADATA(sname, x, __VA_ARGS__) \
> __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
>
> +asmlinkage void verify_pre_usermode_state(void);
> +
> +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> +static inline bool has_user_ds(void) {
> + bool ret = segment_eq(get_fs(), USER_DS);
> + // Prevent re-ordering the call
> + barrier();
> + return ret;
> +}
> +#else
> +static inline bool has_user_ds(void) {
> + return false;
> +}
> +#endif
> +
> +
> #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
> #define __SYSCALL_DEFINEx(x, name, ...) \
> asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
> @@ -199,7 +215,10 @@ extern struct trace_event_functions exit_syscall_print_funcs;
> asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \
> asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
> { \
> + bool user_caller = has_user_ds(); \
> long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
> + if (user_caller) \
> + verify_pre_usermode_state(); \
> __MAP(x,__SC_TEST,__VA_ARGS__); \
> __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
> return ret; \
> diff --git a/init/Kconfig b/init/Kconfig
> index c859c993c26f..ab958b59063f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1929,6 +1929,22 @@ config PROFILING
> config TRACEPOINTS
> bool
>
> +#
> +# Set by each architecture that want to optimize how verify_pre_usermode_state
> +# is called.
> +#
> +config ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
> + bool
> +
> +config VERIFY_PRE_USERMODE_STATE_BUG
> + bool "Halt on incorrect state on returning to user-mode"
> + default n
> + help
> + By default a warning is logged and the state is fixed. This option
> + crashes the kernel instead.
> +
> + If unsure, say Y.
> +
> source "arch/Kconfig"
>
> endmenu # General setup
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 196c7134bee6..cc2ebf7fae55 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2459,3 +2459,14 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
> return 0;
> }
> #endif /* CONFIG_COMPAT */
> +
> +/* Called before coming back to user-mode */
> +asmlinkage void verify_pre_usermode_state(void)
> +{
> +#ifdef CONFIG_VERIFY_PRE_USERMODE_STATE_BUG
> + BUG_ON(!segment_eq(get_fs(), USER_DS));
> +#else
> + if (WARN_ON(!segment_eq(get_fs(), USER_DS)))
> + set_fs(USER_DS);
> +#endif

I would just make this:

if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS))
set_fs(USER_DS);

-Kees

--
Kees Cook
Pixel Security