Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

Daniel Colascione
2018-11-19T15:59:29Z

On Mon, Nov 19, 2018 at 2:32 AM, Christian Brauner <christian@brauner.io> wrote:
> The kill() syscall operates on process identifiers. After a process has
> exited its pid can be reused by another process. If a caller sends a signal
> to a reused pid it will end up signaling the wrong process. This issue has
> often surfaced and there has been a push [1] to address this problem.
>
> A prior patch has introduced the ability to get a file descriptor
> referencing struct pid by opening /proc/<pid>. This guarantees a stable
> handle on a process which can be used to send signals to the referenced
> process. Discussion has shown that a dedicated syscall is preferable over
> ioctl()s. Thus, the new syscall procfd_signal() is introduced to solve
> this problem. It operates on a process file descriptor.
> The syscall takes an additional siginfo_t and flags argument. If siginfo_t
> is NULL then procfd_signal() behaves like kill() if it is not NULL it
> behaves like rt_sigqueueinfo.
> The flags argument is added to allow for future extensions of this syscall.
> It currently needs to be passed as 0.
>
> With this patch a process can be killed via:
>
> #define _GNU_SOURCE
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <signal.h>
> #include <sys/stat.h>
> #include <sys/syscall.h>
> #include <sys/types.h>
> #include <unistd.h>
>
> int main(int argc, char *argv[])
> {
> int ret;
> char buf[1000];
>
> if (argc < 2)
> exit(EXIT_FAILURE);
>
> ret = snprintf(buf, sizeof(buf), "/proc/%s", argv[1]);
> if (ret < 0)
> exit(EXIT_FAILURE);
>
> int fd = open(buf, O_DIRECTORY | O_CLOEXEC);
> if (fd < 0) {
> printf("%s - Failed to open \"%s\"\n", strerror(errno), buf);
> exit(EXIT_FAILURE);
> }
>
> ret = syscall(__NR_procfd_signal, fd, SIGKILL, NULL, 0);
> if (ret < 0) {
> printf("Failed to send SIGKILL \"%s\"\n", strerror(errno));
> close(fd);
> exit(EXIT_FAILURE);
> }
>
> close(fd);
>
> exit(EXIT_SUCCESS);
> }
>
> [1]: https://lkml.org/lkml/2018/11/18/130
>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Serge Hallyn <serge@hallyn.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andy Lutomirsky <luto@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Aleksa Sarai <cyphar@cyphar.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Christian Brauner <christian@brauner.io>
> ---
> Changelog:
> v1:
> - patch introduced
> ---
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> fs/proc/base.c | 6 ++
> include/linux/proc_fs.h | 1 +
> include/linux/syscalls.h | 2 +
> kernel/signal.c | 76 ++++++++++++++++++++++++--
> 6 files changed, 81 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 3cf7b533b3d1..e637eab883e9 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -398,3 +398,4 @@
> 384 i386 arch_prctl sys_arch_prctl __ia32_compat_sys_arch_prctl
> 385 i386 io_pgetevents sys_io_pgetevents __ia32_compat_sys_io_pgetevents
> 386 i386 rseq sys_rseq __ia32_sys_rseq
> +387 i386 procfd_signal sys_procfd_signal __ia32_sys_procfd_signal
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index f0b1709a5ffb..e95f6741ab42 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -343,6 +343,7 @@
> 332 common statx __x64_sys_statx
> 333 common io_pgetevents __x64_sys_io_pgetevents
> 334 common rseq __x64_sys_rseq
> +335 common procfd_signal __x64_sys_procfd_signal
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 6365a4fea314..a12c9de92bd0 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3055,6 +3055,12 @@ static const struct file_operations proc_tgid_base_operations = {
> .release = proc_tgid_release,
> };
>
> +bool proc_is_procfd(const struct file *file)
> +{
> + return d_is_dir(file->f_path.dentry) &&
> + (file->f_op == &proc_tgid_base_operations);
> +}
> +
> static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
> {
> return proc_pident_lookup(dir, dentry,
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index d0e1f1522a78..2d53a47fba34 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -73,6 +73,7 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
> int (*show)(struct seq_file *, void *),
> proc_write_t write,
> void *data);
> +extern bool proc_is_procfd(const struct file *file);
>
> #else /* CONFIG_PROC_FS */
>
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 2ac3d13a915b..a5ca8cb84566 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -907,6 +907,8 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
> unsigned mask, struct statx __user *buffer);
> asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
> int flags, uint32_t sig);
> +asmlinkage long sys_procfd_signal(int fd, int sig, siginfo_t __user *info,
> + int flags);
>
> /*
> * Architecture-specific system calls
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9a32bc2088c9..e8a8929de710 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -19,7 +19,9 @@
> #include <linux/sched/task.h>
> #include <linux/sched/task_stack.h>
> #include <linux/sched/cputime.h>
> +#include <linux/file.h>
> #include <linux/fs.h>
> +#include <linux/proc_fs.h>
> #include <linux/tty.h>
> #include <linux/binfmts.h>
> #include <linux/coredump.h>
> @@ -3286,6 +3288,16 @@ COMPAT_SYSCALL_DEFINE4(rt_sigtimedwait, compat_sigset_t __user *, uthese,
> }
> #endif
>
> +static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
> +{
> + clear_siginfo(info);
> + info->si_signo = sig;
> + info->si_errno = 0;
> + info->si_code = SI_USER;
> + info->si_pid = task_tgid_vnr(current);
> + info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
> +}
> +
> /**
> * sys_kill - send a signal to a process
> * @pid: the PID of the process
> @@ -3295,16 +3307,68 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
> {
> struct kernel_siginfo info;
>
> - clear_siginfo(&info);
> - info.si_signo = sig;
> - info.si_errno = 0;
> - info.si_code = SI_USER;
> - info.si_pid = task_tgid_vnr(current);
> - info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
> + prepare_kill_siginfo(sig, &info);
>
> return kill_something_info(sig, &info, pid);
> }
>
> +/**
> + * sys_procfd_signal - send a signal to a process through a process file
> + * descriptor
> + * @fd: the file descriptor of the process
> + * @sig: signal to be sent
> + * @info: the signal info
> + * @flags: future flags to be passed
> + */
> +SYSCALL_DEFINE4(procfd_signal, int, fd, int, sig, siginfo_t __user *, info,
> + int, flags)
> +{
> + int ret;
> + struct pid *pid;
> + kernel_siginfo_t kinfo;
> + struct fd f;
> +
> + /* Enforce flags be set to 0 until we add an extension. */
> + if (flags)
> + return -EINVAL;
> +
> + f = fdget_raw(fd);
> + if (!f.file)
> + return -EBADF;
> +
> + ret = -EINVAL;
> + /* Is this a process file descriptor? */
> + if (!proc_is_procfd(f.file))
> + goto err;
> +
> + pid = f.file->private_data;

You never addressed my comment on the previous patch about your use of
private_data here. Why can't you use the struct pid reference that's
already in the inode?