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

[Kees Cook <keescook@chromium.org>]

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