um: Implement kernel side of SECCOMP based process handling

This adds the kernel side of the seccomp based process handling.

Co-authored-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Benjamin Berg <benjamin@sipsolutions.net>
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250602130052.545733-6-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

2 files changed, 132 insertions, 15 deletions

diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index 1e146a0f9549..87a18ae4da19 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -40,11 +40,9 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 		list_add(&mm->context.list, &mm_list);
 	}
 
-	new_id->pid = start_userspace(stack);
-	if (new_id->pid < 0) {
-		ret = new_id->pid;
+	ret = start_userspace(new_id);
+	if (ret < 0)
 		goto out_free;
-	}
 
 	/* Ensure the new MM is clean and nothing unwanted is mapped */
 	unmap(new_id, 0, STUB_START);
diff --git a/arch/um/kernel/skas/stub_exe.c b/arch/um/kernel/skas/stub_exe.c
index 23c99b285e82..f40f2332b676 100644
--- a/arch/um/kernel/skas/stub_exe.c
+++ b/arch/um/kernel/skas/stub_exe.c
@@ -3,6 +3,9 @@
 #include <asm/unistd.h>
 #include <sysdep/stub.h>
 #include <stub-data.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <generated/asm-offsets.h>
 
 void _start(void);
 
@@ -25,8 +28,6 @@ noinline static void real_init(void)
 	} sa = {
 		/* Need to set SA_RESTORER (but the handler never returns) */
 		.sa_flags = SA_ONSTACK | SA_NODEFER | SA_SIGINFO | 0x04000000,
-		/* no need to mask any signals */
-		.sa_mask = 0,
 	};
 
 	/* set a nice name */
@@ -35,6 +36,9 @@ noinline static void real_init(void)
 	/* Make sure this process dies if the kernel dies */
 	stub_syscall2(__NR_prctl, PR_SET_PDEATHSIG, SIGKILL);
 
+	/* Needed in SECCOMP mode (and safe to do anyway) */
+	stub_syscall5(__NR_prctl, PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+
 	/* read information from STDIN and close it */
 	res = stub_syscall3(__NR_read, 0,
 			    (unsigned long)&init_data, sizeof(init_data));
@@ -63,18 +67,133 @@ noinline static void real_init(void)
 	stack.ss_sp = (void *)init_data.stub_start + UM_KERN_PAGE_SIZE;
 	stub_syscall2(__NR_sigaltstack, (unsigned long)&stack, 0);
 
-	/* register SIGSEGV handler */
-	sa.sa_handler_ = (void *) init_data.segv_handler;
-	res = stub_syscall4(__NR_rt_sigaction, SIGSEGV, (unsigned long)&sa, 0,
-			    sizeof(sa.sa_mask));
-	if (res != 0)
-		stub_syscall1(__NR_exit, 13);
+	/* register signal handlers */
+	sa.sa_handler_ = (void *) init_data.signal_handler;
+	sa.sa_restorer = (void *) init_data.signal_restorer;
+	if (!init_data.seccomp) {
+		/* In ptrace mode, the SIGSEGV handler never returns */
+		sa.sa_mask = 0;
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGSEGV,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 13);
+	} else {
+		/* SECCOMP mode uses rt_sigreturn, need to mask all signals */
+		sa.sa_mask = ~0ULL;
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGSEGV,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 14);
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGSYS,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 15);
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGALRM,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 16);
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGTRAP,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 17);
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGILL,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 18);
+
+		res = stub_syscall4(__NR_rt_sigaction, SIGFPE,
+				    (unsigned long)&sa, 0, sizeof(sa.sa_mask));
+		if (res != 0)
+			stub_syscall1(__NR_exit, 19);
+	}
+
+	/*
+	 * If in seccomp mode, install the SECCOMP filter and trigger a syscall.
+	 * Otherwise set PTRACE_TRACEME and do a SIGSTOP.
+	 */
+	if (init_data.seccomp) {
+		struct sock_filter filter[] = {
+#if __BITS_PER_LONG > 32
+			/* [0] Load upper 32bit of instruction pointer from seccomp_data */
+			BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+				 (offsetof(struct seccomp_data, instruction_pointer) + 4)),
+
+			/* [1] Jump forward 3 instructions if the upper address is not identical */
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (init_data.stub_start) >> 32, 0, 3),
+#endif
+			/* [2] Load lower 32bit of instruction pointer from seccomp_data */
+			BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+				 (offsetof(struct seccomp_data, instruction_pointer))),
+
+			/* [3] Mask out lower bits */
+			BPF_STMT(BPF_ALU | BPF_AND | BPF_K, 0xfffff000),
+
+			/* [4] Jump to [6] if the lower bits are not on the expected page */
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, (init_data.stub_start) & 0xfffff000, 1, 0),
+
+			/* [5] Trap call, allow */
+			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_TRAP),
+
+			/* [6,7] Check architecture */
+			BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+				 offsetof(struct seccomp_data, arch)),
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K,
+				 UM_SECCOMP_ARCH_NATIVE, 1, 0),
+
+			/* [8] Kill (for architecture check) */
+			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
+
+			/* [9] Load syscall number */
+			BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+				 offsetof(struct seccomp_data, nr)),
+
+			/* [10-14] Check against permitted syscalls */
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_futex,
+				 5, 0),
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, STUB_MMAP_NR,
+				 4, 0),
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_munmap,
+				 3, 0),
+#ifdef __i386__
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_set_thread_area,
+				 2, 0),
+#else
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_arch_prctl,
+				 2, 0),
+#endif
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_rt_sigreturn,
+				 1, 0),
+
+			/* [15] Not one of the permitted syscalls */
+			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
+
+			/* [16] Permitted call for the stub */
+			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
+		};
+		struct sock_fprog prog = {
+			.len = sizeof(filter) / sizeof(filter[0]),
+			.filter = filter,
+		};
+
+		if (stub_syscall3(__NR_seccomp, SECCOMP_SET_MODE_FILTER,
+				  SECCOMP_FILTER_FLAG_TSYNC,
+				  (unsigned long)&prog) != 0)
+			stub_syscall1(__NR_exit, 20);
 
-	stub_syscall4(__NR_ptrace, PTRACE_TRACEME, 0, 0, 0);
+		/* Fall through, the exit syscall will cause SIGSYS */
+	} else {
+		stub_syscall4(__NR_ptrace, PTRACE_TRACEME, 0, 0, 0);
 
-	stub_syscall2(__NR_kill, stub_syscall0(__NR_getpid), SIGSTOP);
+		stub_syscall2(__NR_kill, stub_syscall0(__NR_getpid), SIGSTOP);
+	}
 
-	stub_syscall1(__NR_exit, 14);
+	stub_syscall1(__NR_exit, 30);
 
 	__builtin_unreachable();
 }