Messages in this thread Patch in this message | | | Date | Wed, 31 Oct 2018 14:37:44 +0000 | Subject | [PATCH v2] Implement /proc/pid/kill | From | Daniel Colascione <> |
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Add a simple proc-based kill interface. To use /proc/pid/kill, just write the signal number in base-10 ASCII to the kill file of the process to be killed: for example, 'echo 9 > /proc/$$/kill'.
Semantically, /proc/pid/kill works like kill(2), except that the process ID comes from the proc filesystem context instead of from an explicit system call parameter. This way, it's possible to avoid races between inspecting some aspect of a process and that process's PID being reused for some other process.
Note that only the real user ID that opened a /proc/pid/kill file can write to it; other users get EPERM. This check prevents confused deputy attacks via, e.g., standard output of setuid programs.
With /proc/pid/kill, it's possible to write a proper race-free and safe pkill(1). An approximation follows. A real program might use openat(2), having opened a process's /proc/pid directory explicitly, with the directory file descriptor serving as a sort of "process handle".
#!/bin/bash set -euo pipefail pat=$1 for proc_status in /proc/*/status; do ( cd $(dirname $proc_status) readarray proc_argv -d'' < cmdline if ((${#proc_argv[@]} > 0)) && [[ ${proc_argv[0]} = *$pat* ]]; then echo 15 > kill fi ) || true; done
Signed-off-by: Daniel Colascione <dancol@google.com> ---
Added a real-user-ID check to prevent confused deputy attacks.
fs/proc/base.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 51 insertions(+)
diff --git a/fs/proc/base.c b/fs/proc/base.c index 7e9f07bf260d..74e494f24b28 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -205,6 +205,56 @@ static int proc_root_link(struct dentry *dentry, struct path *path) return result; } +static ssize_t proc_pid_kill_write(struct file *file, + const char __user *buf, + size_t count, loff_t *ppos) +{ + ssize_t res; + int sig; + char buffer[4]; + + /* This check prevents a confused deputy attack in which an + * unprivileged process opens /proc/victim/kill and convinces + * a privileged process to write to that kill FD, effectively + * performing a kill with the privileges of the unwitting + * privileged process. Here, we just fail the kill operation + * if someone calls write(2) with a real user ID that differs + * from the one used to open the kill FD. + */ + res = -EPERM; + if (file->f_cred->user != current_user()) + goto out; + + res = -EINVAL; + if (*ppos != 0) + goto out; + + res = -EINVAL; + if (count > sizeof(buffer) - 1) + goto out; + + res = -EFAULT; + if (copy_from_user(buffer, buf, count)) + goto out; + + buffer[count] = '\0'; + res = kstrtoint(strstrip(buffer), 10, &sig); + if (res) + goto out; + + res = kill_pid(proc_pid(file_inode(file)), sig, 0); + if (res) + goto out; + res = count; +out: + return res; + +} + +static const struct file_operations proc_pid_kill_ops = { + .write = proc_pid_kill_write, +}; + static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf, size_t count, loff_t *ppos) { @@ -2935,6 +2985,7 @@ static const struct pid_entry tgid_base_stuff[] = { #ifdef CONFIG_HAVE_ARCH_TRACEHOOK ONE("syscall", S_IRUSR, proc_pid_syscall), #endif + REG("kill", S_IRUGO | S_IWUGO, proc_pid_kill_ops), REG("cmdline", S_IRUGO, proc_pid_cmdline_ops), ONE("stat", S_IRUGO, proc_tgid_stat), ONE("statm", S_IRUGO, proc_pid_statm), -- 2.19.1.568.g152ad8e336-goog
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