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.
This change also changes the signature of __secure_computing. It
appears the only direct caller is the arm entry code and it clobbers
any possible return value (register) immediately.
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Eric Paris <eparis@redhat.com>
v18: - fix up comments and rebase
- fix bad var name which was fixed in later revs
- remove _int() and just change the __secure_computing signature
v16-v17: ...
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
Signed-off-by: James Morris <james.l.morris@oracle.com>
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 0f7c709..5f78fb6 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -199,15 +199,20 @@
static u32 seccomp_run_filters(int syscall)
{
struct seccomp_filter *f;
- u32 ret = SECCOMP_RET_KILL;
+ u32 ret = SECCOMP_RET_ALLOW;
+
+ /* Ensure unexpected behavior doesn't result in failing open. */
+ if (WARN_ON(current->seccomp.filter == NULL))
+ return SECCOMP_RET_KILL;
+
/*
* All filters in the list are evaluated and the lowest BPF return
- * value always takes priority.
+ * value always takes priority (ignoring the DATA).
*/
for (f = current->seccomp.filter; f; f = f->prev) {
- ret = sk_run_filter(NULL, f->insns);
- if (ret != SECCOMP_RET_ALLOW)
- break;
+ u32 cur_ret = sk_run_filter(NULL, f->insns);
+ if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
+ ret = cur_ret;
}
return ret;
}
@@ -346,11 +351,13 @@
};
#endif
-void __secure_computing(int this_syscall)
+int __secure_computing(int this_syscall)
{
int mode = current->seccomp.mode;
int exit_sig = 0;
int *syscall;
+ u32 ret = SECCOMP_RET_KILL;
+ int data;
switch (mode) {
case SECCOMP_MODE_STRICT:
@@ -361,14 +368,26 @@
#endif
do {
if (*syscall == this_syscall)
- return;
+ return 0;
} while (*++syscall);
exit_sig = SIGKILL;
break;
#ifdef CONFIG_SECCOMP_FILTER
case SECCOMP_MODE_FILTER:
- if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
- return;
+ ret = seccomp_run_filters(this_syscall);
+ data = ret & SECCOMP_RET_DATA;
+ switch (ret & SECCOMP_RET_ACTION) {
+ case SECCOMP_RET_ERRNO:
+ /* Set the low-order 16-bits as a errno. */
+ syscall_set_return_value(current, task_pt_regs(current),
+ -data, 0);
+ goto skip;
+ case SECCOMP_RET_ALLOW:
+ return 0;
+ case SECCOMP_RET_KILL:
+ default:
+ break;
+ }
exit_sig = SIGSYS;
break;
#endif
@@ -379,8 +398,11 @@
#ifdef SECCOMP_DEBUG
dump_stack();
#endif
- audit_seccomp(this_syscall, exit_code, SECCOMP_RET_KILL);
+ audit_seccomp(this_syscall, exit_sig, ret);
do_exit(exit_sig);
+skip:
+ audit_seccomp(this_syscall, exit_sig, ret);
+ return -1;
}
long prctl_get_seccomp(void)