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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index c2c64c1..6075fa6 100644 (file)
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -667,6 +667,14 @@ 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 35e0a31..2ab3ead 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -72,6 +72,33 @@
 #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--;
@@ -874,6 +901,8 @@ void __noreturn do_exit(long code)
 
 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.
@@ -898,6 +927,19 @@ void __noreturn make_task_dead(int signr)
        }
 
        /*
+        * 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);
+
+       /*
         * We're taking recursive faults here in make_task_dead. Safest is to just
         * leave this task alone and wait for reboot.
         */