Re: [PATCH v2] signal: add procfd_signal() syscall

From: Serge E. Hallyn
Date: Thu Nov 22 2018 - 03:00:20 EST

On Tue, Nov 20, 2018 at 11:51:23AM +0100, Christian Brauner 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.
>
> This patch uses file descriptors from proc/<pid> as stable handles on
> struct pid. Even if a pid is recycled the handle will not change. The file
> descriptor can be used to send signals to the referenced process.
> 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)

|| ret > sizeof(buf) ? :-) I mean, you *are* passing the string...

> 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@xxxxxxxxxxxx>
> Cc: Serge Hallyn <serge@xxxxxxxxxx>

Acked-by: Serge Hallyn <serge@xxxxxxxxxx>

> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Andy Lutomirsky <luto@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
> ---
> Changelog:
> v2:
> - define __NR_procfd_signal in unistd.h
> - wire up compat syscall
> - s/proc_is_procfd/proc_is_tgid_procfd/g
> - provide stubs when CONFIG_PROC_FS=n
> - move proc_pid() to linux/proc_fs.h header
> - use proc_pid() to grab struct pid from /proc/<pid> fd
> v1:
> - patch introduced
> ---
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 2 +
> fs/proc/base.c | 11 ++-
> fs/proc/internal.h | 5 -
> include/linux/proc_fs.h | 12 +++
> include/linux/syscalls.h | 2 +
> include/uapi/asm-generic/unistd.h | 4 +-
> kernel/signal.c | 127 +++++++++++++++++++++++--
> 8 files changed, 151 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 3cf7b533b3d1..3f27ffd8ae87 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_compat_sys_procfd_signal
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index f0b1709a5ffb..8a30cde82450 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 64 procfd_signal __x64_sys_procfd_signal
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> @@ -386,3 +387,4 @@
> 545 x32 execveat __x32_compat_sys_execveat/ptregs
> 546 x32 preadv2 __x32_compat_sys_preadv64v2
> 547 x32 pwritev2 __x32_compat_sys_pwritev64v2
> +548 x32 procfd_signal __x32_compat_sys_procfd_signal
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ce3465479447..771c6bd1cac6 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -716,7 +716,10 @@ static int proc_pid_permission(struct inode *inode, int mask)
> return generic_permission(inode, mask);
> }
>
> -
> +struct pid *proc_pid(const struct inode *inode)
> +{
> + return PROC_I(inode)->pid;
> +}
>
> static const struct inode_operations proc_def_inode_operations = {
> .setattr = proc_setattr,
> @@ -3038,6 +3041,12 @@ static const struct file_operations proc_tgid_base_operations = {
> .llseek = generic_file_llseek,
> };
>
> +bool proc_is_tgid_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/fs/proc/internal.h b/fs/proc/internal.h
> index 5185d7f6a51e..eb69afba83f3 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -113,11 +113,6 @@ static inline void *__PDE_DATA(const struct inode *inode)
> return PDE(inode)->data;
> }
>
> -static inline struct pid *proc_pid(const struct inode *inode)
> -{
> - return PROC_I(inode)->pid;
> -}
> -
> static inline struct task_struct *get_proc_task(const struct inode *inode)
> {
> return get_pid_task(proc_pid(inode), PIDTYPE_PID);
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index d0e1f1522a78..96df2fe6311d 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -73,6 +73,8 @@ 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_tgid_procfd(const struct file *file);
> +extern struct pid *proc_pid(const struct inode *inode);
>
> #else /* CONFIG_PROC_FS */
>
> @@ -114,6 +116,16 @@ static inline int remove_proc_subtree(const char *name, struct proc_dir_entry *p
> #define proc_create_net(name, mode, parent, state_size, ops) ({NULL;})
> #define proc_create_net_single(name, mode, parent, show, data) ({NULL;})
>
> +static inline bool proc_is_tgid_procfd(const struct file *file)
> +{
> + return false;
> +}
> +
> +static inline struct pid *proc_pid(const struct inode *inode)
> +{
> + return NULL;
> +}
> +
> #endif /* CONFIG_PROC_FS */
>
> struct net;
> 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/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 538546edbfbd..4dc81a994ad1 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -738,9 +738,11 @@ __SYSCALL(__NR_statx, sys_statx)
> __SC_COMP(__NR_io_pgetevents, sys_io_pgetevents, compat_sys_io_pgetevents)
> #define __NR_rseq 293
> __SYSCALL(__NR_rseq, sys_rseq)
> +#define __NR_procfd_signal 294
> +__SC_COMP(__NR_procfd_signal, sys_procfd_signal, compat_sys_procfd_signal)
>
> #undef __NR_syscalls
> -#define __NR_syscalls 294
> +#define __NR_syscalls 295
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9a32bc2088c9..13695342f150 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 +3307,119 @@ 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);
> }
>
> +/*
> + * Verify that the signaler and signalee either are in the same pid namespace
> + * or that the signaler's pid namespace is an ancestor of the signalee's pid
> + * namespace.
> + */
> +static bool may_signal_procfd(struct pid *pid)
> +{
> + struct pid_namespace *active = task_active_pid_ns(current);
> + struct pid_namespace *p = ns_of_pid(pid);
> +
> + for (;;) {
> + if (!p)
> + return false;
> + if (p == active)
> + break;
> + p = p->parent;
> + }
> +
> + return true;
> +}
> +
> +static int do_procfd_signal(int fd, int sig, kernel_siginfo_t *kinfo, int flags,
> + bool had_siginfo)
> +{
> + int ret;
> + struct fd f;
> + struct pid *pid;
> +
> + /* Enforce flags be set to 0 until we add an extension. */
> + if (flags)
> + return -EINVAL;
> +
> + f = fdget_raw(fd);
> + if (!f.file)
> + return -EBADF;
> +
> + /* Is this a process file descriptor? */
> + ret = -EINVAL;
> + if (!proc_is_tgid_procfd(f.file))
> + goto err;
> +
> + /* Without CONFIG_PROC_FS proc_pid() returns NULL. */
> + pid = proc_pid(file_inode(f.file));
> + if (!pid)
> + goto err;
> +
> + if (!may_signal_procfd(pid))
> + goto err;
> +
> + if (had_siginfo) {
> + /*
> + * Not even root can pretend to send signals from the kernel.
> + * Nor can they impersonate a kill()/tgkill(), which adds
> + * source info.
> + */
> + ret = -EPERM;
> + if ((kinfo->si_code >= 0 || kinfo->si_code == SI_TKILL) &&
> + (task_pid(current) != pid))
> + goto err;
> + } else {
> + prepare_kill_siginfo(sig, kinfo);
> + }
> +
> + ret = kill_pid_info(sig, kinfo, pid);
> +
> +err:
> + fdput(f);
> + return ret;
> +}
> +
> +/**
> + * 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)
> +{
> + kernel_siginfo_t kinfo;
> +
> + if (info) {
> + int ret = __copy_siginfo_from_user(sig, &kinfo, info);
> + if (unlikely(ret))
> + return ret;
> + }
> +
> + return do_procfd_signal(fd, sig, &kinfo, flags, !!info);
> +}
> +
> +#ifdef CONFIG_COMPAT
> +COMPAT_SYSCALL_DEFINE4(procfd_signal, int, fd, int, sig,
> + struct compat_siginfo __user *, info, int, flags)
> +{
> + kernel_siginfo_t kinfo;
> +
> + if (info) {
> + int ret = __copy_siginfo_from_user32(sig, &kinfo, info);
> + if (unlikely(ret))
> + return ret;
> + }
> +
> + return do_procfd_signal(fd, sig, &kinfo, flags, !!info);
> +}
> +#endif
> +
> static int
> do_send_specific(pid_t tgid, pid_t pid, int sig, struct kernel_siginfo *info)
> {
> --
> 2.19.1