[RFC PATCH 5/5] pidfd_mem: implemented remote memory mapping system call

["Adalbert Lazăr" <alazar@bitdefender.com>]

From: Mircea Cirjaliu <mcirjaliu@bitdefender.com>

This system call returns 2 fds for inspecting the address space of a
remote process: one for control and one for access. Use according to
remote mapping specifications.

Cc: Christian Brauner <christian@brauner.io>
Signed-off-by: Mircea Cirjaliu <mcirjaliu@bitdefender.com>
Signed-off-by: Adalbert Lazăr <alazar@bitdefender.com>
---
 arch/x86/entry/syscalls/syscall_32.tbl |  1 +
 arch/x86/entry/syscalls/syscall_64.tbl |  1 +
 include/linux/pid.h                    |  1 +
 include/linux/syscalls.h               |  1 +
 include/uapi/asm-generic/unistd.h      |  2 +
 kernel/exit.c                          |  2 +-
 kernel/pid.c                           | 55 ++++++++++++++++++++++++++
 7 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 54581ac671b4..ca1b5a32dbc5 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -440,5 +440,6 @@
 433	i386	fspick			sys_fspick
 434	i386	pidfd_open		sys_pidfd_open
 435	i386	clone3			sys_clone3
+436     i386    pidfd_mem               sys_pidfd_mem
 437	i386	openat2			sys_openat2
 438	i386	pidfd_getfd		sys_pidfd_getfd
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 37b844f839bc..6138d3d023f8 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -357,6 +357,7 @@
 433	common	fspick			sys_fspick
 434	common	pidfd_open		sys_pidfd_open
 435	common	clone3			sys_clone3
+436     common  pidfd_mem               sys_pidfd_mem
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd		sys_pidfd_getfd
 
diff --git a/include/linux/pid.h b/include/linux/pid.h
index cc896f0fc4e3..9ec23ab23fd4 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -76,6 +76,7 @@ extern const struct file_operations pidfd_fops;
 
 struct file;
 
+extern struct pid *pidfd_get_pid(unsigned int fd);
 extern struct pid *pidfd_pid(const struct file *file);
 
 static inline struct pid *get_pid(struct pid *pid)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1815065d52f3..621f3d52ed4e 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -934,6 +934,7 @@ asmlinkage long sys_clock_adjtime32(clockid_t which_clock,
 asmlinkage long sys_syncfs(int fd);
 asmlinkage long sys_setns(int fd, int nstype);
 asmlinkage long sys_pidfd_open(pid_t pid, unsigned int flags);
+asmlinkage long sys_pidfd_mem(int pidfd, int __user *fds, unsigned int flags);
 asmlinkage long sys_sendmmsg(int fd, struct mmsghdr __user *msg,
 			     unsigned int vlen, unsigned flags);
 asmlinkage long sys_process_vm_readv(pid_t pid,
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 3a3201e4618e..2663afc03c86 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -850,6 +850,8 @@ __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
 #define __NR_clone3 435
 __SYSCALL(__NR_clone3, sys_clone3)
 #endif
+#define __NR_pidfd_mem 436
+__SYSCALL(__NR_pidfd_mem, sys_pidfd_mem)
 
 #define __NR_openat2 437
 __SYSCALL(__NR_openat2, sys_openat2)
diff --git a/kernel/exit.c b/kernel/exit.c
index 389a88cb3081..37cd8949e606 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1464,7 +1464,7 @@ static long do_wait(struct wait_opts *wo)
 	return retval;
 }
 
-static struct pid *pidfd_get_pid(unsigned int fd)
+struct pid *pidfd_get_pid(unsigned int fd)
 {
 	struct fd f;
 	struct pid *pid;
diff --git a/kernel/pid.c b/kernel/pid.c
index c835b844aca7..c9c49edf4a8a 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -42,6 +42,7 @@
 #include <linux/sched/signal.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
+#include <linux/remote_mapping.h>
 
 struct pid init_struct_pid = {
 	.count		= REFCOUNT_INIT(1),
@@ -565,6 +566,60 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 	return fd;
 }
 
+/**
+ * pidfd_mem() - Allow access to process address space.
+ *
+ * @pidfd: pid file descriptor for the target process
+ * @fds:   array where the control and access file descriptors are returned
+ * @flags: flags to pass
+ *
+ * This creates a pair of file descriptors used to gain access to the
+ * target process memory. The control fd is used to establish a linear
+ * mapping between an offset range and a userspace address range.
+ * The access fd is used to mmap(offset range) on the client side.
+ *
+ * Return: On success, 0 is returned.
+ *         On error, a negative errno number will be returned.
+ */
+SYSCALL_DEFINE3(pidfd_mem, int, pidfd, int __user *, fds, unsigned int, flags)
+{
+	struct pid *pid;
+	struct task_struct *task;
+	int ret_fds[2];
+	int ret;
+
+	if (pidfd < 0)
+		return -EINVAL;
+	if (!fds)
+		return -EINVAL;
+	if (flags)
+		return -EINVAL;
+
+	pid = pidfd_get_pid(pidfd);
+	if (IS_ERR(pid))
+		return PTR_ERR(pid);
+
+	task = get_pid_task(pid, PIDTYPE_PID);
+	put_pid(pid);
+	if (IS_ERR(task))
+		return PTR_ERR(task);
+
+	ret = -EPERM;
+	if (unlikely(task == current) || capable(CAP_SYS_PTRACE))
+		ret = task_remote_map(task, ret_fds);
+	put_task_struct(task);
+	if (IS_ERR_VALUE((long)ret))
+		return ret;
+
+	if (copy_to_user(fds, ret_fds, sizeof(ret_fds))) {
+		put_unused_fd(ret_fds[0]);
+		put_unused_fd(ret_fds[1]);
+		return -EFAULT;
+	}
+
+	return 0;
+}
+
 void __init pid_idr_init(void)
 {
 	/* Verify no one has done anything silly: */