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 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 <thgar...@google.com> Tested-by: Kees Cook <keesc...@chromium.org> --- Based on next-20170410 --- arch/s390/Kconfig | 1 + include/linux/syscalls.h | 26 +++++++++++++++++++++++++- init/Kconfig | 6 ++++++ kernel/sys.c | 13 +++++++++++++ 4 files changed, 45 insertions(+), 1 deletion(-) diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig index d25435d94b6e..489a0cc6e46b 100644 --- a/arch/s390/Kconfig +++ b/arch/s390/Kconfig @@ -103,6 +103,7 @@ config S390 select ARCH_INLINE_WRITE_UNLOCK_BH select ARCH_INLINE_WRITE_UNLOCK_IRQ select ARCH_INLINE_WRITE_UNLOCK_IRQRESTORE + select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE select ARCH_SAVE_PAGE_KEYS if HIBERNATION select ARCH_SUPPORTS_ATOMIC_RMW select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index 980c3c9b06f8..801a7a74fe28 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h @@ -191,6 +191,27 @@ 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 verify_pre_usermode_state(void) { + BUG_ON(!segment_eq(get_fs(), USER_DS)); +} + +#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE +#define __CHECK_USER_CALLER() \ + bool user_caller = segment_eq(get_fs(), USER_DS) +#define __VERIFY_PRE_USERMODE_STATE() \ + if (user_caller) verify_pre_usermode_state() +#else +#define __CHECK_USER_CALLER() +#define __VERIFY_PRE_USERMODE_STATE() +asmlinkage void address_limit_check_failed(void); +#endif + + #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__) #define __SYSCALL_DEFINEx(x, name, ...) \ asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \ @@ -199,7 +220,10 @@ 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; \ + __CHECK_USER_CALLER(); \ + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \ + __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 7f7027817bce..e5fbd0becfa7 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1958,6 +1958,12 @@ config PROFILING config TRACEPOINTS bool +config ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE + bool + help + Disable the generic pre-usermode state verification. 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 196c7134bee6..d30530ff8166 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2459,3 +2459,16 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info) return 0; } #endif /* CONFIG_COMPAT */ + +#ifdef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE +/* + * This function is called when an architecture specific implementation detected + * an invalid address limit. The generic user-mode state checker will finish on + * the appropriate BUG_ON. + */ +asmlinkage void address_limit_check_failed(void) +{ + verify_pre_usermode_state(); + panic("address_limit_check_failed called with a valid user-mode state"); +} +#endif -- 2.12.2.715.g7642488e1d-goog
