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

[Thomas Garnier]

On Fri, Apr 28, 2017 at 8:32 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> 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@xxxxxxxxxx>
> Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>

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