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SubjectRe: [PATCH v5 2/3] pid: Introduce pidfd_getfd syscall
On 2019-12-20, Sargun Dhillon <sargun@sargun.me> wrote:
> This syscall allows for the retrieval of file descriptors from other
> processes, based on their pidfd. This is possible using ptrace, and
> injection of parasitic code along with using SCM_RIGHTS to move
> file descriptors between a tracee and a tracer. Unfortunately, ptrace
> comes with a high cost of requiring the process to be stopped, and
> breaks debuggers. This does not require stopping the process under
> manipulation.
>
> One reason to use this is to allow sandboxers to take actions on file
> descriptors on the behalf of another process. For example, this can be
> combined with seccomp-bpf's user notification to do on-demand fd
> extraction and take privileged actions. For example, it can be used
> to bind a socket to a privileged port.
>
> /* prototype */
> /*
> * pidfd_getfd_options is an extensible struct which can have options
> * added to it. If options is NULL, size, and it will be ignored be
> * ignored, otherwise, size should be set to sizeof(*options). If
> * option is newer than the current kernel version, E2BIG will be
> * returned.
> */
> struct pidfd_getfd_options {};
> long pidfd_getfd(int pidfd, int fd, unsigned int flags,
> struct pidfd_getfd_options *options, size_t size);
>
> /* testing */
> Ran self-test suite on x86_64
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
> MAINTAINERS | 1 +
> arch/alpha/kernel/syscalls/syscall.tbl | 1 +
> arch/arm/tools/syscall.tbl | 1 +
> arch/arm64/include/asm/unistd.h | 2 +-
> arch/arm64/include/asm/unistd32.h | 2 +
> arch/ia64/kernel/syscalls/syscall.tbl | 1 +
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
> arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
> arch/parisc/kernel/syscalls/syscall.tbl | 1 +
> arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
> arch/s390/kernel/syscalls/syscall.tbl | 1 +
> arch/sh/kernel/syscalls/syscall.tbl | 1 +
> arch/sparc/kernel/syscalls/syscall.tbl | 1 +
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
> include/linux/syscalls.h | 4 +
> include/uapi/asm-generic/unistd.h | 3 +-
> include/uapi/linux/pidfd.h | 10 ++
> kernel/pid.c | 115 ++++++++++++++++++++
> 23 files changed, 151 insertions(+), 2 deletions(-)
> create mode 100644 include/uapi/linux/pidfd.h
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index cc0a4a8ae06a..bc370ff59dbf 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13014,6 +13014,7 @@ M: Christian Brauner <christian@brauner.io>
> L: linux-kernel@vger.kernel.org
> S: Maintained
> T: git git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git
> +F: include/uapi/linux/pidfd.h
> F: samples/pidfd/
> F: tools/testing/selftests/pidfd/
> F: tools/testing/selftests/clone3/
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index 8e13b0b2928d..d1cac0d657b7 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -475,3 +475,4 @@
> 543 common fspick sys_fspick
> 544 common pidfd_open sys_pidfd_open
> # 545 reserved for clone3
> +548 common pidfd_getfd sys_pidfd
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index 6da7dc4d79cc..ba045e2f3a60 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -449,3 +449,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> 435 common clone3 sys_clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 2629a68b8724..b722e47377a5 100644
> --- a/arch/arm64/include/asm/unistd.h
> +++ b/arch/arm64/include/asm/unistd.h
> @@ -38,7 +38,7 @@
> #define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
> #define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
>
> -#define __NR_compat_syscalls 436
> +#define __NR_compat_syscalls 439
> #endif
>
> #define __ARCH_WANT_SYS_CLONE
> diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
> index 94ab29cf4f00..a8da97a2de41 100644
> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -879,6 +879,8 @@ __SYSCALL(__NR_fspick, sys_fspick)
> __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
> #define __NR_clone3 435
> __SYSCALL(__NR_clone3, sys_clone3)
> +#define __NR_pidfd_getfd 438
> +__SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
>
> /*
> * Please add new compat syscalls above this comment and update
> diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> index 36d5faf4c86c..2b11adfc860c 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -356,3 +356,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> # 435 reserved for clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index a88a285a0e5f..44e879e98459 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -435,3 +435,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> # 435 reserved for clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index 09b0cd7dab0a..7afa00125cc4 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -441,3 +441,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> 435 common clone3 sys_clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
> index e7c5ab38e403..856d5ba34461 100644
> --- a/arch/mips/kernel/syscalls/syscall_n32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
> @@ -374,3 +374,4 @@
> 433 n32 fspick sys_fspick
> 434 n32 pidfd_open sys_pidfd_open
> 435 n32 clone3 __sys_clone3
> +438 n32 pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
> index 13cd66581f3b..2db6075352f3 100644
> --- a/arch/mips/kernel/syscalls/syscall_n64.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
> @@ -350,3 +350,4 @@
> 433 n64 fspick sys_fspick
> 434 n64 pidfd_open sys_pidfd_open
> 435 n64 clone3 __sys_clone3
> +438 n64 pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
> index 353539ea4140..e9f9d4a9b105 100644
> --- a/arch/mips/kernel/syscalls/syscall_o32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
> @@ -423,3 +423,4 @@
> 433 o32 fspick sys_fspick
> 434 o32 pidfd_open sys_pidfd_open
> 435 o32 clone3 __sys_clone3
> +438 o32 pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index 285ff516150c..c58c7eb144ca 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -433,3 +433,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> 435 common clone3 sys_clone3_wrapper
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index 43f736ed47f2..707609bfe3ea 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -517,3 +517,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> 435 nospu clone3 ppc_clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 3054e9c035a3..185cd624face 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -438,3 +438,4 @@
> 433 common fspick sys_fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open sys_pidfd_open
> 435 common clone3 sys_clone3 sys_clone3
> +438 common pidfd_getfd sys_pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index b5ed26c4c005..88f90895aad8 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -438,3 +438,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> # 435 reserved for clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index 8c8cc7537fb2..218df6a2326e 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -481,3 +481,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> # 435 reserved for clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 15908eb9b17e..9c3101b65e0f 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -440,3 +440,4 @@
> 433 i386 fspick sys_fspick __ia32_sys_fspick
> 434 i386 pidfd_open sys_pidfd_open __ia32_sys_pidfd_open
> 435 i386 clone3 sys_clone3 __ia32_sys_clone3
> +438 i386 pidfd_getfd sys_pidfd_getfd __ia32_sys_pidfd_getfd
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index c29976eca4a8..cef85db75a62 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -357,6 +357,7 @@
> 433 common fspick __x64_sys_fspick
> 434 common pidfd_open __x64_sys_pidfd_open
> 435 common clone3 __x64_sys_clone3/ptregs
> +438 common pidfd_getfd __x64_sys_pidfd_getfd
>
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index 25f4de729a6d..ae15183def12 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -406,3 +406,4 @@
> 433 common fspick sys_fspick
> 434 common pidfd_open sys_pidfd_open
> 435 common clone3 sys_clone3
> +438 common pidfd_getfd sys_pidfd_getfd
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 2960dedcfde8..62fe706329d1 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -69,6 +69,7 @@ struct rseq;
> union bpf_attr;
> struct io_uring_params;
> struct clone_args;
> +struct pidfd_getfd_options;
>
> #include <linux/types.h>
> #include <linux/aio_abi.h>
> @@ -1000,6 +1001,9 @@ asmlinkage long sys_fspick(int dfd, const char __user *path, unsigned int flags)
> asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
> siginfo_t __user *info,
> unsigned int flags);
> +asmlinkage long sys_pidfd_getfd(int pidfd, int fd,
> + struct pidfd_getfd_options __user *options,
> + size_t, usize);
>
> /*
> * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 1fc8faa6e973..f358488366f6 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -850,9 +850,10 @@ __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
> #define __NR_clone3 435
> __SYSCALL(__NR_clone3, sys_clone3)
> #endif
> +#define __NR_pidfd_getfd 438
>
> #undef __NR_syscalls
> -#define __NR_syscalls 436
> +#define __NR_syscalls 439
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
> new file mode 100644
> index 000000000000..0a3fc922661d
> --- /dev/null
> +++ b/include/uapi/linux/pidfd.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _UAPI_LINUX_PIDFD_H
> +#define _UAPI_LINUX_PIDFD_H
> +
> +struct pidfd_getfd_options {};

Are empty structs well-defined in C (from memory, some compilers make
them non-zero in size)? Since we probably plan to add a flags field in
the future anyway, why not just have a __u64 flags which must be zeroed?

> +
> +#define PIDFD_GETFD_OPTIONS_SIZE_VER0 0
> +#define PIDFD_GETFD_OPTIONS_SIZE_LATEST PIDFD_GETFD_OPTIONS_SIZE_VER0
> +
> +#endif /* _UAPI_LINUX_PIDFD_H */
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 2278e249141d..2a9cb4be383f 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 <uapi/linux/pidfd.h>
>
> struct pid init_struct_pid = {
> .count = REFCOUNT_INIT(1),
> @@ -578,3 +579,117 @@ void __init pid_idr_init(void)
> init_pid_ns.pid_cachep = KMEM_CACHE(pid,
> SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
> }
> +
> +static struct file *__pidfd_getfd_fget_task(struct task_struct *task, u32 fd)
> +{
> + struct file *file;
> + int ret;
> +
> + ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
> + if (ret)
> + return ERR_PTR(ret);
> +
> + if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS)) {
> + file = ERR_PTR(-EPERM);
> + goto out;
> + }
> +
> + file = fget_task(task, fd);
> + if (!file)
> + file = ERR_PTR(-EBADF);
> +
> +out:
> + mutex_unlock(&task->signal->cred_guard_mutex);
> + return file;
> +}
> +
> +static long pidfd_getfd(struct pid *pid, u32 fd)
> +{
> + struct task_struct *task;
> + struct file *file;
> + int ret, retfd;
> +
> + task = get_pid_task(pid, PIDTYPE_PID);
> + if (!task)
> + return -ESRCH;
> +
> + file = __pidfd_getfd_fget_task(task, fd);
> + put_task_struct(task);
> + if (IS_ERR(file))
> + return PTR_ERR(file);
> +
> + retfd = get_unused_fd_flags(O_CLOEXEC);
> + if (retfd < 0) {
> + ret = retfd;
> + goto out;
> + }
> +
> + /*
> + * security_file_receive must come last since it may have side effects
> + * and cannot be reversed.
> + */
> + ret = security_file_receive(file);
> + if (ret)
> + goto out_put_fd;
> +
> + fd_install(retfd, file);
> + return retfd;
> +
> +out_put_fd:
> + put_unused_fd(retfd);
> +out:
> + fput(file);
> + return ret;
> +}
> +
> +/**
> + * sys_pidfd_getfd() - Get a file descriptor from another process
> + *
> + * @pidfd: file descriptor of the process
> + * @fd: the file descriptor number to get
> + * @options: options on how to get the fd
> + * @usize: the size of options
> + *
> + * This syscall requires that the process has the ability to ptrace the
> + * process represented by the pidfd. It will return a duplicated version
> + * of the file descriptor on success. The process who which is having
> + * its file descriptor taken is otherwise unaffected. If options is NULL
> + * it is ignored along with usize.
> + *
> + * Return: On success, a file descriptor with cloexec is returned.
> + * On error, a negative errno number will be returned.
> + */
> +SYSCALL_DEFINE4(pidfd_getfd, int, pidfd, int, fd,
> + struct pidfd_getfd_options __user *, options, size_t, usize)
> +{
> + struct pid *pid;
> + struct fd f;
> + int ret;
> +
> + BUILD_BUG_ON(sizeof(struct pidfd_getfd_options) != PIDFD_GETFD_OPTIONS_SIZE_LATEST);
> +
> + /*
> + * options is currently unused, verify it's unset or if it is set,
> + * ensure that size is 0.
> + *
> + * In the future, this will need to adopt copy_struct_from_user.
> + */
> + if (options && usize > PIDFD_GETFD_OPTIONS_SIZE_VER0)
> + return -E2BIG;

I wouldn't suggest doing this. I understand that this is simpler, but it
will cause problems once we add a field to pidfd_getfd_options -- newer
programs compiled with a larger struct won't work on older kernels with
this check (even if the struct is completely zeroed).

If copy_struct_from_user() doesn't work with usize == 0, I would *much*
prefer a patch which fixes that (effectively copy_struct_from_user()
with a usize == 0 should just be a call to check_zeroed_user()).

> + f = fdget(pidfd);
> + if (!f.file)
> + return -EBADF;
> +
> + pid = pidfd_pid(f.file);
> + if (IS_ERR(pid)) {
> + ret = PTR_ERR(pid);
> + goto out;
> + }
> +
> + ret = pidfd_getfd(pid, fd);
> +
> +out:
> + fdput(f);
> + return ret;
> +}
> --
> 2.20.1

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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