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SubjectRe: [PATCH v6 2/3] pid: Introduce pidfd_getfd syscall
On Mon, Dec 23, 2019 at 09:09:08PM +0000, Sargun Dhillon 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. One such privileged action
> can be to bind a socket to a privileged port.
>
> This also adds the syscall to all architectures at the same time.
>
> /* prototype */
> /* flags is currently reserved and should be set to 0 */
> long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
>
> /* testing */
> Ran self-test suite on x86_64
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
> 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 | 1 +
> include/uapi/asm-generic/unistd.h | 3 +-
> kernel/pid.c | 106 ++++++++++++++++++++
> 21 files changed, 128 insertions(+), 2 deletions(-)
>
> 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..5edbc31af51f 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1000,6 +1000,7 @@ 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, unsigned int flags);
>
> /*
> * 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

This looks wrong. This should be:

#define __NR_pidfd_getfd 438
__SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)

>
> #undef __NR_syscalls
> -#define __NR_syscalls 436
> +#define __NR_syscalls 439
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 2278e249141d..8f65468cd857 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -578,3 +578,109 @@ 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)

Hm, you're using int fd in the actual syscall, the fget_task() helper
uses an unsigned int and the new helpers you add here use u32. That's a
lot of inconsistency. :)
Looking through the syscalls we have
"unsigned long fd",
"unsigned int fd",
"int fd"
I suggest to either use "unsigned int fd" or "int fd". I don't
particularly care which it is. glibc's wrappers usually expose all fds
as ints to userspace so I'm not sure if there's any benefit for us in
using unsigned int. So I guess "int fd" everywhere up until fget_task()
is reasonable:
static struct file *__pidfd_getfd_fget_task(struct task_struct *task, int fd)
static long pidfd_getfd(struct pid *pid, int fd)
SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd, unsigned int, flags)
Also, please simply name this function __pidfd_fget()

> +{
> + 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)

You're returning a long here but are using "int" everywhere. Thus I'd
make this:
static int pidfd_getfd(struct pid *pid, int fd)
which also makes it consistent with how it is used in the syscall itself.

> +{
> + 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);

+1

> + 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: the pidfd file descriptor of the process
> + * @fd: the file descriptor number to get
> + * @flags: flags on how to get the fd (reserved)
> + *
> + * This syscall gets a copy of a file descriptor from another process
> + * based on their pidfd, and file descriptor number. It requires that
> + * the calling process has the ability to ptrace the process represented
> + * by the pidfd. The process which is having its file descriptor copied
> + * is otherwise unaffected.
> + *
> + * Return: On success, a file descriptor with cloexec is returned.

"On success, a cloexec file descriptor is returned." reads better, I
think.

> + * On error, a negative errno number will be returned.
> + */
> +SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
> + unsigned int, flags)
> +{
> + struct pid *pid;
> + struct fd f;
> + int ret;
> +
> + /* flags is currently unused - make sure it's unset */
> + if (flags)
> + return -EINVAL;
> +
> + 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;

This can be simplified to:

pid = pidfd_pid(f.file);
if (IS_ERR(pid))
ret = PTR_ERR(pid);
else
ret = pidfd_getfd(pid, fd);
fdput(f);
return ret;

Christian

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