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

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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: 307d522f5e ("signal/seccomp: Refactor seccomp signal and coredump generation")
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2021-10-29 09:14:19 -05:00
parent e21294a7aa
commit 00b06da29c
3 changed files with 11 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/signal_types.h
View File

@ -70,6 +70,9 @@ struct ksignal {
int sig;
};
/* Used to kill the race between sigaction and forced signals */
#define SA_IMMUTABLE 0x00800000
#ifndef __ARCH_UAPI_SA_FLAGS
#ifdef SA_RESTORER
#define __ARCH_UAPI_SA_FLAGS SA_RESTORER

1
include/uapi/asm-generic/signal-defs.h
View File

@ -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 */
/*

8
kernel/signal.c
View File

@ -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 @@ relock:
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;