seccomp: Add SECCOMP_RET_TRAP

Adds a new return value to seccomp filters that triggers a SIGSYS to be
delivered with the new SYS_SECCOMP si_code.

This allows in-process system call emulation, including just specifying
an errno or cleanly dumping core, rather than just dying.

Suggested-by: Markus Gutschke <markus@chromium.org>
Suggested-by: Julien Tinnes <jln@chromium.org>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Eric Paris <eparis@redhat.com>

v18: - acked-by, rebase
     - don't mention secure_computing_int() anymore
v15: - use audit_seccomp/skip
     - pad out error spacing; clean up switch (indan@nul.nu)
v14: - n/a
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - rebase on to linux-next
v11: - clarify the comment (indan@nul.nu)
     - s/sigtrap/sigsys
v10: - use SIGSYS, syscall_get_arch, updates arch/Kconfig
       note suggested-by (though original suggestion had other behaviors)
v9:  - changes to SIGILL
v8:  - clean up based on changes to dependent patches
v7:  - introduction
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 5f78fb6d2212..9c3830692a08 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -332,6 +332,26 @@ void put_seccomp_filter(struct task_struct *tsk)
                 kfree(freeme);
         }
 }
+
+/**
+ * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
+ * @syscall: syscall number to send to userland
+ * @reason: filter-supplied reason code to send to userland (via si_errno)
+ *
+ * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
+ */
+static void seccomp_send_sigsys(int syscall, int reason)
+{
+        struct siginfo info;
+        memset(&info, 0, sizeof(info));
+        info.si_signo = SIGSYS;
+        info.si_code = SYS_SECCOMP;
+        info.si_call_addr = (void __user *)KSTK_EIP(current);
+        info.si_errno = reason;
+        info.si_arch = syscall_get_arch(current, task_pt_regs(current));
+        info.si_syscall = syscall;
+        force_sig_info(SIGSYS, &info, current);
+}
 #endif  /* CONFIG_SECCOMP_FILTER */
 
 /*
@@ -382,6 +402,12 @@ int __secure_computing(int this_syscall)
                         syscall_set_return_value(current, task_pt_regs(current),
                                                  -data, 0);
                         goto skip;
+                case SECCOMP_RET_TRAP:
+                        /* Show the handler the original registers. */
+                        syscall_rollback(current, task_pt_regs(current));
+                        /* Let the filter pass back 16 bits of data. */
+                        seccomp_send_sigsys(this_syscall, data);
+                        goto skip;
                 case SECCOMP_RET_ALLOW:
                         return 0;
                 case SECCOMP_RET_KILL: