Commit 28facdf7 authored Feb 01, 2023 by Jann Horn Committed by Greg Kroah-Hartman Feb 06, 2023

exit: Put an upper limit on how often we can oops

commit d4ccd54d28d3c8598e2354acc13e28c060961dbb upstream.

Many Linux systems are configured to not panic on oops; but allowing an
attacker to oops the system **really** often can make even bugs that look
completely unexploitable exploitable (like NULL dereferences and such) if
each crash elevates a refcount by one or a lock is taken in read mode, and
this causes a counter to eventually overflow.

The most interesting counters for this are 32 bits wide (like open-coded
refcounts that don't use refcount_t). (The ldsem reader count on 32-bit
platforms is just 16 bits, but probably nobody cares about 32-bit platforms
that much nowadays.)

So let's panic the system if the kernel is constantly oopsing.

The speed of oopsing 2^32 times probably depends on several factors, like
how long the stack trace is and which unwinder you're using; an empirically
important one is whether your console is showing a graphical environment or
a text console that oopses will be printed to.
In a quick single-threaded benchmark, it looks like oopsing in a vfork()
child with a very short stack trace only takes ~510 microseconds per run
when a graphical console is active; but switching to a text console that
oopses are printed to slows it down around 87x, to ~45 milliseconds per
run.
(Adding more threads makes this faster, but the actual oops printing
happens under &die_lock on x86, so you can maybe speed this up by a factor
of around 2 and then any further improvement gets eaten up by lock
contention.)

It looks like it would take around 8-12 days to overflow a 32-bit counter
with repeated oopsing on a multi-core X86 system running a graphical
environment; both me (in an X86 VM) and Seth (with a distro kernel on
normal hardware in a standard configuration) got numbers in that ballpark.

12 days aren't *that* short on a desktop system, and you'd likely need much
longer on a typical server system (assuming that people don't run graphical
desktop environments on their servers), and this is a *very* noisy and
violent approach to exploiting the kernel; and it also seems to take orders
of magnitude longer on some machines, probably because stuff like EFI
pstore will slow it down a ton if that's active.

Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20221107201317.324457-1-jannh@google.com

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221117234328.594699-2-keescook@chromium.org

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 3fa431ba

Documentation/admin-guide/sysctl/kernel.rst

+8 −0

Original line number	Diff line number	Diff line
		@@ -557,6 +557,14 @@ numa_balancing_scan_size_mb is how many megabytes worth of pages are
		scanned for a given scan.


		oops_limit
		==========

		Number of kernel oopses after which the kernel should panic when
		``panic_on_oops`` is not set. Setting this to 0 or 1 has the same effect
		as setting ``panic_on_oops=1``.


		osrelease, ostype & version:
		============================

kernel/exit.c

+43 −0

Original line number	Diff line number	Diff line
		@@ -69,6 +69,33 @@
		#include <asm/pgtable.h>
		#include <asm/mmu_context.h>

		/*
		* The default value should be high enough to not crash a system that randomly
		* crashes its kernel from time to time, but low enough to at least not permit
		* overflowing 32-bit refcounts or the ldsem writer count.
		*/
		static unsigned int oops_limit = 10000;

		#ifdef CONFIG_SYSCTL
		static struct ctl_table kern_exit_table[] = {
		{
		.procname = "oops_limit",
		.data = &oops_limit,
		.maxlen = sizeof(oops_limit),
		.mode = 0644,
		.proc_handler = proc_douintvec,
		},
		{ }
		};

		static __init int kernel_exit_sysctls_init(void)
		{
		register_sysctl_init("kernel", kern_exit_table);
		return 0;
		}
		late_initcall(kernel_exit_sysctls_init);
		#endif

		static void __unhash_process(struct task_struct *p, bool group_dead)
		{
		nr_threads--;
		@@ -866,10 +893,26 @@ EXPORT_SYMBOL_GPL(do_exit);

		void __noreturn make_task_dead(int signr)
		{
		static atomic_t oops_count = ATOMIC_INIT(0);

		/*
		* Take the task off the cpu after something catastrophic has
		* happened.
		*/

		/*
		* Every time the system oopses, if the oops happens while a reference
		* to an object was held, the reference leaks.
		* If the oops doesn't also leak memory, repeated oopsing can cause
		* reference counters to wrap around (if they're not using refcount_t).
		* This means that repeated oopsing can make unexploitable-looking bugs
		* exploitable through repeated oopsing.
		* To make sure this can't happen, place an upper bound on how often the
		* kernel may oops without panic().
		*/
		if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);

		do_exit(signr);
		}