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@xxxxxxxxxx> --- I picked 10000 here to also provide safety for the ldsem code on 32-bit systems, but you could also argue that the real fix there is to make ldsem more robust, and that the limit should be something like 2^31... An alternative approach would be to always let make_task_dead() take the do_task_dead() path and never exit; but that would probably be a more disruptive change? @Kees should this go through your tree? (After waiting a while for the inevitable bikeshedding on whether the default limit should be closer to 10000 or 2^31.) Documentation/admin-guide/sysctl/kernel.rst | 7 +++++++ include/linux/panic.h | 1 + kernel/exit.c | 22 +++++++++++++++++++++ kernel/sysctl.c | 7 +++++++ 4 files changed, 37 insertions(+) diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index 98d1b198b2b4c..09713f25b3d62 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -667,6 +667,13 @@ 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. + + osrelease, ostype & version =========================== diff --git a/include/linux/panic.h b/include/linux/panic.h index c7759b3f20452..5b3e029fe1eb0 100644 --- a/include/linux/panic.h +++ b/include/linux/panic.h @@ -21,6 +21,7 @@ extern int panic_on_oops; extern int panic_on_unrecovered_nmi; extern int panic_on_io_nmi; extern int panic_on_warn; +extern int oops_limit; extern unsigned long panic_on_taint; extern bool panic_on_taint_nousertaint; diff --git a/kernel/exit.c b/kernel/exit.c index 35e0a31a0315c..827ceffbfa432 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -872,8 +872,17 @@ void __noreturn do_exit(long code) do_task_dead(); } +/* + * 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. + */ +int oops_limit = 10000; + 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. @@ -897,6 +906,19 @@ void __noreturn make_task_dead(int signr) preempt_count_set(PREEMPT_ENABLED); } + /* + * 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 (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. diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 188c305aeb8b7..63370aa4c078f 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1866,6 +1866,13 @@ static struct ctl_table kern_table[] = { .mode = 0644, .proc_handler = proc_dointvec, }, + { + .procname = "oops_limit", + .data = &oops_limit, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec, + }, { .procname = "panic_print", .data = &panic_print, base-commit: f0c4d9fc9cc9462659728d168387191387e903cc -- 2.38.1.431.g37b22c650d-goog