Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 03/09/2017 02:24 AM, Thomas Garnier wrote:
> This patch ensures a syscall does not return to user-mode with a kernel
> address limit. If that happened, a process can corrupt kernel-mode
> memory and elevate privileges.
> 
> For example, it would mitigation this bug:
> 
> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
> 
> If the CONFIG_BUG_ON_DATA_CORRUPTION option is enabled, an incorrect
> state will result in a BUG_ON.
> 
> The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
> added so each architecture can optimize this change.
> 
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
> Based on next-20170308
> ---
>  include/linux/syscalls.h | 19 +++++++++++++++++++
>  init/Kconfig             |  7 +++++++
>  kernel/sys.c             |  8 ++++++++
>  3 files changed, 34 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;
> +}

Can you please disable that for s390?  (e.g. by setting 
CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE for s390)
We have a separate address space for kernel/user so the logic will
be slightly different and is already handled in

commit b5a882fcf146c87cb6b67c6df353e1c042b8773d
Author: Heiko Carstens <heiko.carstens@de.ibm.com>
Date:   Fri Feb 17 08:13:28 2017 +0100

    s390: restore address space when returning to user space



> +#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..c4efc3a95e4a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1929,6 +1929,13 @@ 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
> +
>  source "arch/Kconfig"
> 
>  endmenu		# General setup
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 196c7134bee6..411163ac9dc3 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2459,3 +2459,11 @@ 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)
> +{
> +	if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
> +				  "incorrect get_fs() on user-mode return"))
> +		set_fs(USER_DS);
> +}
>