UBUNTU: SAUCE: SECCOMP: seccomp: add SECCOMP_RET_ERRNO

This change adds the SECCOMP_RET_ERRNO as a valid return value from a
seccomp filter.  Additionally, it makes the first use of the lower
16-bits for storing a filter-supplied errno.  16-bits is more than
enough for the errno-base.h calls.

Returning errors instead of immediately terminating processes that
violate seccomp policy allow for broader use of this functionality
for kernel attack surface reduction.  For example, a linux container
could maintain a whitelist of pre-existing system calls but drop
all new ones with errnos.  This would keep a logically static attack
surface while providing errnos that may allow for graceful failure
without the downside of do_exit() on a bad call.

v15: - use audit_seccomp and add a skip label. (eparis@redhat.com)
     - clean up and pad out return codes (indan@nul.nu)
v14: - no change/rebase
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - move to WARN_ON if filter is NULL
       (oleg@redhat.com, luto@mit.edu, keescook@chromium.org)
     - return immediately for filter==NULL (keescook@chromium.org)
     - change evaluation to only compare the ACTION so that layered
       errnos don't result in the lowest one being returned.
       (keeschook@chromium.org)
v11: - check for NULL filter (keescook@chromium.org)
v10: - change loaders to fn
 v9: - n/a
 v8: - update Kconfig to note new need for syscall_set_return_value.
     - reordered such that TRAP behavior follows on later.
     - made the for loop a little less indent-y
 v7: - introduced

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Signed-off-by: Kees Cook <kees@ubuntu.com>

1 files changed, 11 insertions, 4 deletions

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index ce980a8ee87..12bb2e31ed1 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -12,13 +12,14 @@
 
 /*
  * All BPF programs must return a 32-bit value.
- * The bottom 16-bits are reserved for future use.
+ * The bottom 16-bits are for optional return data.
  * The upper 16-bits are ordered from least permissive values to most.
  *
  * The ordering ensures that a min_t() over composed return values always
  * selects the least permissive choice.
  */
 #define SECCOMP_RET_KILL	0x00000000U /* kill the task immediately */
+#define SECCOMP_RET_ERRNO	0x00050000U /* returns an errno */
 #define SECCOMP_RET_ALLOW	0x7fff0000U /* allow */
 
 /* Masks for the return value sections. */
@@ -64,11 +65,17 @@ struct seccomp {
 	struct seccomp_filter *filter;
 };
 
-extern void __secure_computing(int);
-static inline void secure_computing(int this_syscall)
+/*
+ * Direct callers to __secure_computing should be updated as
+ * CONFIG_HAVE_ARCH_SECCOMP_FILTER propagates.
+ */
+extern void __secure_computing(int) __deprecated;
+extern int __secure_computing_int(int);
+static inline int secure_computing(int this_syscall)
 {
 	if (unlikely(test_thread_flag(TIF_SECCOMP)))
-		__secure_computing(this_syscall);
+		return  __secure_computing_int(this_syscall);
+	return 0;
 }
 
 extern long prctl_get_seccomp(void);