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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id lw16sm9718884pjb.51.2022.02.14.12.07.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Feb 2022 12:07:25 -0800 (PST) Date: Mon, 14 Feb 2022 12:07:25 -0800 From: Kees Cook To: Christian Brauner Cc: Andy Lutomirski , Robert =?utf-8?B?xZp3acSZY2tp?= , "Eric W. Biederman" , Jann Horn , Will Drewry , linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [RFC] Get siginfo from unreaped task Message-ID: <202202141152.6296CD7F@keescook> References: <20220213085212.cwzuqlrabpgbnbac@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220213085212.cwzuqlrabpgbnbac@wittgenstein> Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On Sun, Feb 13, 2022 at 09:52:12AM +0100, Christian Brauner wrote: > On Sat, Feb 12, 2022 at 06:32:08PM -0800, Andy Lutomirski wrote: > > > > > On Feb 12, 2022, at 3:24 AM, Robert Święcki wrote: > > > > > > sob., 12 lut 2022 o 05:28 Kees Cook napisał(a): > > >> > > >> Make siginfo available through PTRACE_GETSIGINFO after process death, > > >> without needing to have already used PTRACE_ATTACH. Uses 48 more bytes > > >> in task_struct, though I bet there might be somewhere else we could > > >> stash a copy of it? > > > > > > An alternative way of accessing this info could be abusing the > > > waitid() interface, with some additional, custom to Linux, flag > > > > > > waitid(P_ALL, 0, &si, __WCHILDSIGINFO); > > > > > > which would change what is put into si. > > > > > > But maybe ptrace() is better, because it's mostly incompatible with > > > other OSes anyway on the behavior/flag level, while waitd() seems to > > > be POSIX/BSD standard, even if Linux specifies some additional flags. > > > > > > > > > > I had a kind of opposite thought, which is that it would be very nice > > to be able to get all the waitid() data without reaping a process or > > even necessarily being its parent. Maybe these can be combined? A > > new waitid() option like you’re suggesting could add siginfo (and > > might need permissions). And we could have a different waitid() flag > > that says “maybe not my child, don’t reap” (and also needs > > permissions). > > > > Although the “don’t reap” thing is fundamentally racy. What a sane > > process manager actually wants is an interface to read all this info > > from a pidfd, which means it all needs to get stuck in struct pid. And > > /me briefly pops out from vacation > > Agreed and not just siginfo I would expect(?). We already came to that > conclusion when we first introduced them. > > > task_struct needs a completion or wait queue so you can actually wait > > for a pidfd to exit (unless someone already did this — I had patches a > > while back). And this would be awesome. > > Currently, you can wait for a pidfd to exit via polling and you can use > a pidfd to pass it to waitid(P_PIDFD, pidfd, ...). > > /me pops back into vacation Right, so waitid already has all the infrastructure for this, so I think adding it there makes a lot of sense. Here's what I've got: 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/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); 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 d54efddd378b..70ecb996cecd 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; }; 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; @@ -1638,6 +1645,10 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *, if (!infop) return err; + /* __WCHILDSIGINFO */ + if (info->siginfo.signo) + return copy_siginfo_to_user(infop, &info->siginfo); + if (!user_write_access_begin(infop, sizeof(*infop))) return -EFAULT; @@ -1781,6 +1792,12 @@ COMPAT_SYSCALL_DEFINE5(waitid, if (!infop) return err; + /* __WCHILDSIGINFO */ + if (info->siginfo.signo) + return copy_siginfo_to_user32( + (struct compat_siginfo __user *)infop, + &info->siginfo); + if (!user_write_access_begin(infop, sizeof(*infop))) return -EFAULT; One usability question I have is: - if the process just exited normally, should it return an empty siginfo, or should it ignore __WCHILDSIGINFO? (I have it ignoring it above.) -- Kees Cook