[PATCH 24/27] um: Add SECCOMP support detection and initialization
Benjamin Berg
benjamin at sipsolutions.net
Wed Mar 3 15:55:20 GMT 2021
This detects seccomp support, sets the global using_seccomp variable and
initilizes the exec registers. For now, the implementation simply falls
through to the ptrace startup code, meaning that it is unused.
Signed-off-by: Benjamin Berg <benjamin at sipsolutions.net>
---
arch/um/include/shared/skas/skas.h | 6 ++
arch/um/os-Linux/registers.c | 4 +-
arch/um/os-Linux/skas/process.c | 3 +
arch/um/os-Linux/start_up.c | 132 ++++++++++++++++++++++++++++-
4 files changed, 141 insertions(+), 4 deletions(-)
diff --git a/arch/um/include/shared/skas/skas.h b/arch/um/include/shared/skas/skas.h
index c93d2cbc8f32..f10599995d4d 100644
--- a/arch/um/include/shared/skas/skas.h
+++ b/arch/um/include/shared/skas/skas.h
@@ -6,8 +6,14 @@
#ifndef __SKAS_H
#define __SKAS_H
+#include <linux/kconfig.h>
#include <sysdep/ptrace.h>
+#ifdef CONFIG_UML_SECCOMP
+extern int using_seccomp;
+#else
+#define using_seccomp 0
+#endif
extern int userspace_pid[];
extern int user_thread(unsigned long stack, int flags);
diff --git a/arch/um/os-Linux/registers.c b/arch/um/os-Linux/registers.c
index 52823368e15a..7c86e0414db0 100644
--- a/arch/um/os-Linux/registers.c
+++ b/arch/um/os-Linux/registers.c
@@ -13,8 +13,8 @@
/* This is set once at boot time and not changed thereafter */
-static unsigned long exec_regs[MAX_REG_NR];
-static unsigned long exec_fp_regs[FP_SIZE];
+unsigned long exec_regs[MAX_REG_NR];
+unsigned long exec_fp_regs[FP_SIZE];
int init_registers(int pid)
{
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 3bfc28d28fc0..a3f0dd0f47d0 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -251,6 +251,9 @@ static int userspace_tramp(void *stack)
return 0;
}
+#ifdef CONFIG_UML_SECCOMP
+int using_seccomp;
+#endif
int userspace_pid[NR_CPUS];
int kill_userspace_mm[NR_CPUS];
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 8e6f8c4f3f62..2a581ea1b0ef 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -1,8 +1,10 @@
// SPDX-License-Identifier: GPL-2.0
/*
+ * Copyright (C) 2021 Benjamin Berg <benjamin at sipsolutions.net>
* Copyright (C) 2000 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
*/
+#include <linux/kconfig.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
@@ -22,6 +24,13 @@
#include <os.h>
#include <mem_user.h>
#include <ptrace_user.h>
+#ifdef CONFIG_UML_SECCOMP
+#include <stub-data.h>
+#include <sys/prctl.h>
+#include <linux/seccomp.h>
+#include <linux/filter.h>
+#include <sysdep/mcontext.h>
+#endif
#include <registers.h>
#include <skas.h>
@@ -221,6 +230,112 @@ static void __init check_ptrace(void)
check_sysemu();
}
+#ifdef CONFIG_UML_SECCOMP
+extern unsigned long exec_regs[MAX_REG_NR];
+extern unsigned long exec_fp_regs[FP_SIZE];
+
+static struct stub_data __initdata *seccomp_shared_page;
+
+static void __init sigsys_handler(int sig, siginfo_t *info, void *p)
+{
+ ucontext_t *uc = p;
+
+ /* Stow away the location of the mcontext in the stack */
+ seccomp_shared_page->mctx_offset = (unsigned long)&uc->uc_mcontext -
+ (unsigned long)&seccomp_shared_page->sigstack[0];
+ exit(0);
+}
+
+static bool __init init_seccomp(void)
+{
+ int pid;
+ int status;
+ int n;
+
+ /* We check that we can install a seccomp filter and then exit(0)
+ * from a trapped syscall.
+ *
+ * Note that we cannot verify that no seccomp filter already exists
+ * for a syscall that results in the process/thread to be killed.
+ */
+
+ os_info("Checking that seccomp filters can be installed...");
+
+ seccomp_shared_page = mmap(0, UM_KERN_PAGE_SIZE,
+ PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, 0, 0);
+
+ pid = fork();
+ if (pid == 0) {
+ static struct sock_filter filter[] = {
+ BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+ offsetof(struct seccomp_data, nr)),
+ BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_exit_group, 0, 1),
+ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
+ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_TRAP),
+ };
+ static struct sock_fprog prog = {
+ .len = ARRAY_SIZE(filter),
+ .filter = filter,
+ };
+ struct sigaction sa;
+
+ set_sigstack(seccomp_shared_page->sigstack,
+ sizeof(seccomp_shared_page->sigstack));
+
+ sa.sa_flags = SA_ONSTACK | SA_NODEFER | SA_SIGINFO;
+ sa.sa_sigaction = (void *) sigsys_handler;
+ sa.sa_restorer = NULL;
+ if (sigaction(SIGSYS, &sa, NULL) < 0)
+ exit(1);
+
+ prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+ if (syscall(__NR_seccomp, SECCOMP_SET_MODE_FILTER,
+ SECCOMP_FILTER_FLAG_TSYNC, &prog) != 0)
+ exit(2);
+
+ sleep(0);
+
+ /* Never reached. */
+ exit(3);
+ }
+
+ if (pid < 0)
+ fatal_perror("check_seccomp : fork failed");
+
+ CATCH_EINTR(n = waitpid(pid, &status, 0));
+ if (n < 0)
+ fatal_perror("check_seccomp : waitpid failed");
+
+ if (WIFEXITED(status) && WEXITSTATUS(status) == 0) {
+ struct uml_pt_regs regs = { 0 };
+
+ /* XXX: Can we add something like this easily anywhere?
+ * COMPILE_BUG_ON(sizeof(*mc->fpregs) > sizeof(regs->fp));
+ */
+
+ /* Copy registers, the init_registers function assumes ptrace. */
+ get_stub_state(®s, seccomp_shared_page);
+
+ memcpy(exec_regs, regs.gp, sizeof(exec_regs));
+ memcpy(exec_fp_regs, regs.fp, sizeof(exec_fp_regs));
+
+ munmap(seccomp_shared_page, sizeof(mcontext_t));
+
+ os_info("OK\n");
+
+ return true;
+ }
+
+ if (WIFEXITED(status) && WEXITSTATUS(status) == 2)
+ os_info("missing\n");
+ else
+ os_info("error\n");
+
+ munmap(seccomp_shared_page, sizeof(mcontext_t));
+ return false;
+}
+#endif
+
extern void check_tmpexec(void);
static void __init check_coredump_limit(void)
@@ -253,13 +368,26 @@ void __init os_early_checks(void)
/* Print out the core dump limits early */
check_coredump_limit();
- check_ptrace();
-
/* Need to check this early because mmapping happens before the
* kernel is running.
*/
check_tmpexec();
+#ifdef CONFIG_UML_SECCOMP
+ using_seccomp = 0;
+
+ if (init_seccomp()) {
+ /* Not fully implemented */
+#if 0
+ using_seccomp = 1;
+
+ return;
+#endif
+ }
+#endif
+
+ check_ptrace();
+
pid = start_ptraced_child();
if (init_registers(pid))
fatal("Failed to initialize default registers");
--
2.29.2
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