On 25/01/23 12:21 am, Eric Biggers wrote:

From: Jann Horn jannh@google.com

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://urldefense.com/v3/__https://lore.kernel.org/r/20221107201317.324457-... Reviewed-by: Luis Chamberlain mcgrof@kernel.org Signed-off-by: Kees Cook keescook@chromium.org Link: https://urldefense.com/v3/__https://lore.kernel.org/r/20221117234328.594699-... Signed-off-by: Eric Biggers ebiggers@google.com

---

Documentation/admin-guide/sysctl/kernel.rst | 8 ++++ kernel/exit.c | 43 +++++++++++++++++++++ 2 files changed, 51 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index 609b891754081..b6e68d6f297e5 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -671,6 +671,14 @@ This is the default behavior. an oops event is detected. +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

diff --git a/kernel/exit.c b/kernel/exit.c index 5d1a507fd4bae..172d7f835f801 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -69,6 +69,33 @@ #include <asm/unistd.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--;

@@ -879,10 +906,26 @@ 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); }

Hi,

Thanks for the backports.

I have tried backporting the oops_limit patches to LTS 5.15.y and had a similar set of patches, just want to add a note here on an alternate way for backporting this patch without resolving conflicts manually:

Here is the sequence:

* Patch 12: [panic: Separate sysctl logic from CONFIG_SMP] --> Cherry-pick Commit: 05ea0424f0e2 ("exit: Move oops specific logic from do_exit into make_task_dead") upstream --> Cherry-pick Commit: de77c3a5b95c ("exit: Move force_uaccess back into do_exit") upstream * Patch 13 which is Commit: d4ccd54d28d3 ("exit: Put an upper limit on how often we can oops") upstream, will be a clean cherry-pick.

The benefit may be making future backports simpler in make_task_dead().

This was the only difference, so your backport looks good to me.

Regards, Harshit