[PATCH] signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed

Eric W. Biederman
2021-10-29T15:09:44Z

As Andy pointed out that there are races between
force_sig_info_to_task and sigaction[1] when force_sig_info_task. As
Kees discovered[2] ptrace is also able to change these signals.

In the case of seeccomp killing a process with a signal it is a
security violation to allow the signal to be caught or manipulated.

Solve this problem by introducing a new flag SA_IMMUTABLE that
prevents sigaction and ptrace from modifying these forced signals.
This flag is carefully made kernel internal so that no new ABI is
introduced.

Longer term I think this can be solved by guaranteeing short circuit
delivery of signals in this case. Unfortunately reliable and
guaranteed short circuit delivery of these signals is still a ways off
from being implemented, tested, and merged. So I have implemented a much
simpler alternative for now.

[1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com
[2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook
Cc: stable@vger.kernel.org
Fixes: 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---

I have tested this patch and this changed works for me to fix the issue.

I believe this closes all of the races that force_sig_info_to_task
has when sigdfl is specified. So this should be enough for anything
that needs a guaranteed that userspace can not race with the kernel
is handled.

Can folks look this over and see if I missed something?
Thank you,
Eric

include/linux/signal_types.h | 3 +++
include/uapi/asm-generic/signal-defs.h | 1 +
kernel/signal.c | 8 +++++++-
3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
index 34cb28b8f16c..927f7c0e5bff 100644
--- a/include/linux/signal_types.h
+++ b/include/linux/signal_types.h
@@ -70,6 +70,9 @@ struct ksignal {
int sig;
};

+/* Used to kill the race between sigaction and forced signals */
+#define SA_IMMUTABLE 0x008000000
+
#ifndef __ARCH_UAPI_SA_FLAGS
#ifdef SA_RESTORER
#define __ARCH_UAPI_SA_FLAGS SA_RESTORER
diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
index fe929e7b77ca..7572f2f46ee8 100644
--- a/include/uapi/asm-generic/signal-defs.h
+++ b/include/uapi/asm-generic/signal-defs.h
@@ -45,6 +45,7 @@
#define SA_UNSUPPORTED 0x00000400
#define SA_EXPOSE_TAGBITS 0x00000800
/* 0x00010000 used on mips */
+/* 0x00800000 used for internal SA_IMMUTABLE */
/* 0x01000000 used on x86 */
/* 0x02000000 used on x86 */
/*
diff --git a/kernel/signal.c b/kernel/signal.c
index 6a5e1802b9a2..056a107e3cbc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1336,6 +1336,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
blocked = sigismember(&t->blocked, sig);
if (blocked || ignored || sigdfl) {
action->sa.sa_handler = SIG_DFL;
+ action->sa.sa_flags |= SA_IMMUTABLE;
if (blocked) {
sigdelset(&t->blocked, sig);
recalc_sigpending_and_wake(t);
@@ -2760,7 +2761,8 @@ bool get_signal(struct ksignal *ksig)
if (!signr)
break; /* will return 0 */

- if (unlikely(current->ptrace) && signr != SIGKILL) {
+ if (unlikely(current->ptrace) && (signr != SIGKILL) &&
+ !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) {
signr = ptrace_signal(signr, &ksig->info);
if (!signr)
continue;
@@ -4110,6 +4112,10 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
k = &p->sighand->action[sig-1];

spin_lock_irq(&p->sighand->siglock);
+ if (k->sa.sa_flags & SA_IMMUTABLE) {
+ spin_unlock_irq(&p->sighand->siglock);
+ return -EINVAL;
+ }
if (oact)
*oact = *k;

--
2.20.1