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.... [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;