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