Re: [PATCH 5.15 13/20] exit: Put an upper limit on how often we can oops

[Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>]

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/20221107201317.324457-1-jannh@google.com__;!!ACWV5N9M2RV99hQ!N-JMN1iGq4TzLl-KgssGXKoBeTEyN5-Qqf4WKpkP9dPj5DpMQejZFXq92OuEL0fWts4dfsuyqTLPWHXVEhx3tDFCvFE$
> 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/20221117234328.594699-2-keescook@chromium.org__;!!ACWV5N9M2RV99hQ!N-JMN1iGq4TzLl-KgssGXKoBeTEyN5-Qqf4WKpkP9dPj5DpMQejZFXq92OuEL0fWts4dfsuyqTLPWHXVEhx3qFbFrr8$
> 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