[RFC PATCH 4/6] um: forbid mmap/munmap in the stub seccomp filter

[Cong Wang]

From: Cong Wang <cwang at multikernel.io>

In seccomp mode the monitor now installs all guest mappings via
pidfd_mmap(), so the stub has no legitimate need to mmap/munmap after
its filter is in place (the pre-filter bootstrap mmaps are unaffected).
Drop the STUB_MMAP_NR and __NR_munmap allowlist arms and fix the jump
offsets; both now hit SECCOMP_RET_KILL_PROCESS. A hijacked stub can no
longer map arbitrary physmem. The filter exists only in seccomp mode,
so ptrace (SKAS0) is untouched.

Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Cong Wang <cwang at multikernel.io>
---
 arch/um/kernel/skas/stub_exe.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/um/kernel/skas/stub_exe.c b/arch/um/kernel/skas/stub_exe.c
index cbafaa684e66..a88274449168 100644
--- a/arch/um/kernel/skas/stub_exe.c
+++ b/arch/um/kernel/skas/stub_exe.c
@@ -165,16 +165,18 @@ noinline static void real_init(void)
 			BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
 				 offsetof(struct seccomp_data, nr)),
 
-			/* [10-16] Check against permitted syscalls */
+			/*
+			 * [10-14] Check against permitted syscalls. mmap and
+			 * munmap are deliberately absent: in seccomp mode the
+			 * monitor installs all guest mappings via pidfd_mmap(),
+			 * so a stub that attempts mmap/munmap hits the KILL at
+			 * [15].
+			 */
 			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_futex,
-				 7, 0),
-			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K,__NR_recvmsg,
-				 6, 0),
-			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K,__NR_close,
 				 5, 0),
-			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, STUB_MMAP_NR,
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_recvmsg,
 				 4, 0),
-			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_munmap,
+			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_close,
 				 3, 0),
 #ifdef __i386__
 			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_set_thread_area,
@@ -186,10 +188,10 @@ noinline static void real_init(void)
 			BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_rt_sigreturn,
 				 1, 0),
 
-			/* [17] Not one of the permitted syscalls */
+			/* [15] Not one of the permitted syscalls */
 			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),
 
-			/* [18] Permitted call for the stub */
+			/* [16] Permitted call for the stub */
 			BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
 		};
 		struct sock_fprog prog = {
-- 
2.43.0