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Message-ID: <20220215091400.m4koiz4qqhgctnlx@wittgenstein>
Date: Tue, 15 Feb 2022 10:14:00 +0100
From: Christian Brauner <brauner@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>,
Andy Lutomirski <luto@...capital.net>,
Robert Święcki <robert@...ecki.net>,
Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] exit: Introduce __WCHILDSIGINFO for waitid
On Mon, Feb 14, 2022 at 01:38:22PM -0800, Kees Cook wrote:
> When __WCHILDSIGINFO is specified, copy the child's siginfo into the
> waitid infop instead of constructing the parent's SIGCHLD perspective.
>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: "Robert Święcki" <robert@...ecki.net>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: linux-api@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> include/linux/sched.h | 1 +
> include/uapi/linux/wait.h | 1 +
> kernel/exit.c | 23 +++++++++++++++++++----
> kernel/signal.c | 4 ++++
> 4 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f5b2be39a78c..e40789e801ef 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1178,6 +1178,7 @@ struct task_struct {
> #endif
> /* Ptrace state: */
> unsigned long ptrace_message;
> + kernel_siginfo_t death_siginfo;
> kernel_siginfo_t *last_siginfo;
>
> struct task_io_accounting ioac;
> diff --git a/include/uapi/linux/wait.h b/include/uapi/linux/wait.h
> index 85b809fc9f11..7258cd4510ba 100644
> --- a/include/uapi/linux/wait.h
> +++ b/include/uapi/linux/wait.h
> @@ -9,6 +9,7 @@
> #define WCONTINUED 0x00000008
> #define WNOWAIT 0x01000000 /* Don't reap, just poll status. */
>
> +#define __WCHILDSIGINFO 0x10000000 /* Report child's siginfo. */
> #define __WNOTHREAD 0x20000000 /* Don't wait on children of other threads in this group */
> #define __WALL 0x40000000 /* Wait on all children, regardless of type */
> #define __WCLONE 0x80000000 /* Wait only on non-SIGCHLD children */
> diff --git a/kernel/exit.c b/kernel/exit.c
> index b00a25bb4ab9..de6e024976c6 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -953,6 +953,7 @@ struct waitid_info {
> uid_t uid;
> int status;
> int cause;
> + kernel_siginfo_t siginfo;
Heh, I once got yelled at because I put siginfo in there... :)
If this time it's acceptable it might be worth considering replacing
some of the duplicated fields if possible. But again, on a suboptimal
device because of vacation.
> };
>
> struct wait_opts {
> @@ -964,7 +965,7 @@ struct wait_opts {
> int wo_stat;
> struct rusage *wo_rusage;
>
> - wait_queue_entry_t child_wait;
> + wait_queue_entry_t child_wait;
> int notask_error;
> };
>
> @@ -1012,11 +1013,16 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
> int state, status;
> pid_t pid = task_pid_vnr(p);
> uid_t uid = from_kuid_munged(current_user_ns(), task_uid(p));
> - struct waitid_info *infop;
> + struct waitid_info *infop = wo->wo_info;
>
> if (!likely(wo->wo_flags & WEXITED))
> return 0;
>
> + /* Before WNOWAIT so a copy can be extracted without reaping. */
> + if (unlikely(wo->wo_flags & __WCHILDSIGINFO)) {
> + if (infop && p->last_siginfo)
> + copy_siginfo(&infop->siginfo, p->last_siginfo);
> + }
> if (unlikely(wo->wo_flags & WNOWAIT)) {
> status = (p->signal->flags & SIGNAL_GROUP_EXIT)
> ? p->signal->group_exit_code : p->exit_code;
> @@ -1121,7 +1127,6 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
> release_task(p);
>
> out_info:
> - infop = wo->wo_info;
> if (infop) {
> if ((status & 0x7f) == 0) {
> infop->cause = CLD_EXITED;
> @@ -1564,7 +1569,7 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
> unsigned int f_flags = 0;
>
> if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED|
> - __WNOTHREAD|__WCLONE|__WALL))
> + __WNOTHREAD|__WCLONE|__WALL|__WCHILDSIGINFO))
> return -EINVAL;
> if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
> return -EINVAL;
> @@ -1637,6 +1642,10 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *,
> if (!infop)
> return err;
>
> + /* __WCHILDSIGINFO */
> + if (info.siginfo.si_signo)
> + return copy_siginfo_to_user(infop, &info.siginfo);
> +
> if (!user_write_access_begin(infop, sizeof(*infop)))
> return -EFAULT;
>
> @@ -1780,6 +1789,12 @@ COMPAT_SYSCALL_DEFINE5(waitid,
> if (!infop)
> return err;
>
> + /* __WCHILDSIGINFO */
> + if (info.siginfo.si_signo)
> + return copy_siginfo_to_user32(
> + (struct compat_siginfo __user *)infop,
> + &info.siginfo);
> +
Just to repeat what I said in the earlier thread: if I'm not misreading
this - I have to do this on a suboptimal device because of vacation -
then there's a behavioral change for what fields are initialized and
what additional fields (if any) are zeroed between a __WCHILDSIGINFO
request and a regular waitid() request. I'm a) not sure we want this and
b) we should document it.
> if (!user_write_access_begin(infop, sizeof(*infop)))
> return -EFAULT;
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9b04631acde8..41f6ba6b7aa7 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2825,6 +2825,10 @@ bool get_signal(struct ksignal *ksig)
> }
>
> fatal:
> + /* Allow siginfo to be queried until reaped. */
> + copy_siginfo(¤t->death_siginfo, &ksig->info);
> + current->last_siginfo = ¤t->death_siginfo;
> +
> spin_unlock_irq(&sighand->siglock);
> if (unlikely(cgroup_task_frozen(current)))
> cgroup_leave_frozen(true);
> --
> 2.30.2
>
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