[RFC PATCH 2/6] um: acquire a stub pidfd via CLONE_PIDFD in seccomp mode

[Cong Wang <xiyou.wangcong@gmail.com>]

From: Cong Wang <cwang@multikernel.io>

In SECCOMP mode the monitor will install guest mappings into the stub's
address space directly via pidfd_mmap(), so it needs a pidfd to the
stub. Acquire one atomically at clone() time via CLONE_PIDFD (returned
through the legacy-clone parent_tid argument), store it in
mm_id->stub_pidfd, and close it on teardown. The ptrace (SKAS0) mode
does not use pidfd_mmap and leaves stub_pidfd as -1.

Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Cong Wang <cwang@multikernel.io>
---
 arch/um/include/shared/skas/mm_id.h |  1 +
 arch/um/kernel/skas/mmu.c           |  6 ++++++
 arch/um/os-Linux/skas/process.c     | 24 ++++++++++++++++++++++--
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/um/include/shared/skas/mm_id.h b/arch/um/include/shared/skas/mm_id.h
index 18c0621430d2..cec97189f12b 100644
--- a/arch/um/include/shared/skas/mm_id.h
+++ b/arch/um/include/shared/skas/mm_id.h
@@ -16,6 +16,7 @@ struct mm_id {
 	int syscall_data_len;
 
 	/* Only used with SECCOMP mode */
+	int stub_pidfd;		/* pidfd to the stub, or -1 */
 	int sock;
 	int syscall_fd_num;
 	int syscall_fd_map[STUB_MAX_FDS];
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index b5017096028b..441dcf94ec9c 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -54,6 +54,7 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 		goto out;
 
 	new_id->stack = stack;
+	new_id->stub_pidfd = -1;
 	new_id->syscall_data_len = 0;
 	new_id->syscall_fd_num = 0;
 
@@ -103,6 +104,11 @@ void destroy_context(struct mm_struct *mm)
 		mmu->id.pid = -1;
 	}
 
+	if (mmu->id.stub_pidfd >= 0) {
+		os_close_file(mmu->id.stub_pidfd);
+		mmu->id.stub_pidfd = -1;
+	}
+
 	if (using_seccomp && mmu->id.sock)
 		os_close_file(mmu->id.sock);
 
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index d6c22f8aa06d..3dd97ca7999a 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -34,6 +34,10 @@
 #include <asm-generic/rwonce.h>
 #include "../internal.h"
 
+#ifndef CLONE_PIDFD
+#define CLONE_PIDFD 0x00001000
+#endif
+
 int is_skas_winch(int pid, int fd, void *data)
 {
 	return pid == getpgrp();
@@ -448,6 +452,9 @@ int start_userspace(struct mm_id *mm_id)
 	void *stack;
 	unsigned long sp;
 	int status, n, err;
+	int stub_pidfd = -1;
+
+	mm_id->stub_pidfd = -1;
 
 	/* setup a temporary stack page */
 	stack = mmap(NULL, UM_KERN_PAGE_SIZE,
@@ -474,15 +481,25 @@ int start_userspace(struct mm_id *mm_id)
 	if (using_seccomp)
 		proc_data->futex = FUTEX_IN_CHILD;
 
+	/*
+	 * In SECCOMP mode, acquire a pidfd to the stub via CLONE_PIDFD (it is
+	 * returned through the legacy-clone parent_tid argument). The monitor
+	 * installs guest mappings into the stub's mm directly via pidfd_mmap(),
+	 * so the stub itself never needs the mmap capability. The ptrace mode
+	 * does not use it and drives the stub directly.
+	 */
 	mm_id->pid = clone(userspace_tramp, (void *) sp,
-		    CLONE_VFORK | CLONE_VM | SIGCHLD,
-		    (void *)&tramp_data);
+		    CLONE_VFORK | CLONE_VM | (using_seccomp ? CLONE_PIDFD : 0) |
+		    SIGCHLD,
+		    (void *)&tramp_data, &stub_pidfd);
 	if (mm_id->pid < 0) {
 		err = -errno;
 		printk(UM_KERN_ERR "%s : clone failed, errno = %d\n",
 		       __func__, errno);
 		goto out_close;
 	}
+	if (using_seccomp)
+		mm_id->stub_pidfd = stub_pidfd;
 
 	if (using_seccomp) {
 		wait_stub_done_seccomp(mm_id, 1, 1);
@@ -534,8 +551,11 @@ int start_userspace(struct mm_id *mm_id)
 out_close:
 	close(tramp_data.sockpair[0]);
 	close(tramp_data.sockpair[1]);
+	if (stub_pidfd >= 0)
+		close(stub_pidfd);
 
 	mm_id->pid = -1;
+	mm_id->stub_pidfd = -1;
 
 	return err;
 }
-- 
2.43.0