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Date: Tue, 12 Jan 2016 20:25:45 +0100
From: Jann Horn <jann@...jh.net>
To: kernel-hardening@...ts.openwall.com,
linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Baoquan He <bhe@...hat.com>,
Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: [RFC] kernel/panic: place an upper limit on number of oopses
To prevent an attacker from turning a mostly harmless oops into an
exploitable issue using a refcounter wraparound caused by repeated
oopsing, limit the number of oopses.
I have not experimentally verified whether the attack I describe
in the comment works, but I don't see why it wouldn't.
(f_count increments through fget() use atomic_long_inc_not_zero(),
but get_file() just does a normal increment and is e.g.
used by dup_fd().)
This approach is strictly inferior to PAX_REFCOUNT, but as long
as that's not upstreamed and turned on by default, it might make
sense to at least use this patch.
Opinions?
Signed-off-by: Jann Horn <jann@...jh.net>
---
kernel/panic.c | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/kernel/panic.c b/kernel/panic.c
index 4b150bc..27a480d 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -422,9 +422,37 @@ void print_oops_end_marker(void)
*/
void oops_exit(void)
{
+ static atomic_t oops_counter = ATOMIC_INIT(0);
+
do_oops_enter_exit();
print_oops_end_marker();
kmsg_dump(KMSG_DUMP_OOPS);
+
+ /*
+ * Every time the system oopses, if the oops happens while a
+ * reference to an object was held (e.g. in a VFS function),
+ * the reference leaks. If the oops doesn't also leak memory,
+ * repeated oopsing can cause the reference counter to wrap
+ * around - in particular, on 32bit systems, f_count in
+ * struct file is only 32 bits long and can realistically
+ * wrap around.
+ * This means that an oops, even if it's just caused by an
+ * unexploitable-looking NULL pointer dereference or so,
+ * could maybe be turned into a use-after-free through a
+ * counter overincrement, and a use-after-free might be
+ * exploitable.
+ * To reduce the probability that this happens, place an
+ * upper bound on how often the kernel may oops - after this
+ * limit is reached, just panic.
+ * The constant used as limit should be low enough to
+ * mitigate this kind of exploitation attempt, but high
+ * enough to avoid unnecessary panics.
+ */
+ if (atomic_inc_return(&oops_counter) >= 0x100000 &&
+ panic_on_oops == 0) {
+ pr_emerg("oopsed too often, setting panic_on_oops=1\n");
+ panic_on_oops = 1;
+ }
}
#ifdef WANT_WARN_ON_SLOWPATH
--
2.1.4
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