[kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

Thomas Garnier
2017-05-05T22:18:30Z

On Fri, Apr 28, 2017 at 8:32 AM, Thomas Garnier <thgarnie@google.com> wrote:
> Ensure that a syscall does not return to user-mode with a kernel address
> limit. If that happens, a process can corrupt kernel-mode memory and
> elevate privileges [1].
>
> The CONFIG_ADDR_LIMIT_CHECK option disables the generic check so each
> architecture can create optimized versions. This option is enabled by
> default on s390 because a similar feature already exists.
>
> [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> Tested-by: Kees Cook <keescook@chromium.org>

Ingo: Do you want to take the set?

> ---
> Based on next-20170426
> ---
> arch/s390/Kconfig | 1 +
> include/linux/syscalls.h | 27 ++++++++++++++++++++++++++-
> init/Kconfig | 6 ++++++
> kernel/sys.c | 13 +++++++++++++
> 4 files changed, 46 insertions(+), 1 deletion(-)
>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index d25435d94b6e..3d2ec084d5fc 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -64,6 +64,7 @@ config ARCH_SUPPORTS_UPROBES
>
> config S390
> def_bool y
> + select ADDR_LIMIT_CHECK
> select ARCH_HAS_DEVMEM_IS_ALLOWED
> select ARCH_HAS_ELF_RANDOMIZE
> select ARCH_HAS_GCOV_PROFILE_ALL
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 980c3c9b06f8..e534b93ce43a 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -191,6 +191,28 @@ extern struct trace_event_functions exit_syscall_print_funcs;
> SYSCALL_METADATA(sname, x, __VA_ARGS__) \
> __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
>
> +
> +/*
> + * Called before coming back to user-mode. Returning to user-mode with an
> + * address limit different than USER_DS can allow to overwrite kernel memory.
> + */
> +static inline void addr_limit_check_syscall(void)
> +{
> + BUG_ON(!segment_eq(get_fs(), USER_DS));
> +}
> +
> +#ifndef CONFIG_ADDR_LIMIT_CHECK
> +#define ADDR_LIMIT_CHECK_PRE() \
> + bool user_caller = segment_eq(get_fs(), USER_DS)
> +#define ADDR_LIMIT_CHECK_POST() \
> + if (user_caller) addr_limit_check_syscall()
> +#else
> +#define ADDR_LIMIT_CHECK_PRE()
> +#define ADDR_LIMIT_CHECK_POST()
> +asmlinkage void addr_limit_check_failed(void) __noreturn;
> +#endif
> +
> +
> #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
> #define __SYSCALL_DEFINEx(x, name, ...) \
> asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
> @@ -199,7 +221,10 @@ 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__)) \
> { \
> - long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
> + long ret; \
> + ADDR_LIMIT_CHECK_PRE(); \
> + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
> + ADDR_LIMIT_CHECK_POST(); \
> __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 42a346b0df43..599d9fe30703 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1961,6 +1961,12 @@ config PROFILING
> config TRACEPOINTS
> bool
>
> +config ADDR_LIMIT_CHECK
> + bool
> + help
> + Disable the generic address limit check. Allow each architecture to
> + optimize how and when the verification is done.
> +
> source "arch/Kconfig"
>
> endmenu # General setup
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 8a94b4eabcaa..a1cbcd715d62 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2458,3 +2458,16 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
> return 0;
> }
> #endif /* CONFIG_COMPAT */
> +
> +#ifdef CONFIG_ADDR_LIMIT_CHECK
> +/*
> + * Used when an architecture specific implementation detects an invalid address
> + * limit. This function does not return.
> + */
> +asmlinkage void addr_limit_check_failed(void)
> +{
> + /* Try to fail on the generic address limit check */
> + addr_limit_check_syscall();
> + panic("Invalid address limit before returning to user-mode");
> +}
> +#endif
> --
> 2.13.0.rc0.306.g87b477812d-goog
>

--
Thomas