This is a note to let you know that I've just added the patch titled exit: Put an upper limit on how often we can oops to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: exit-put-an-upper-limit-on-how-often-we-can-oops.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From stable-owner@xxxxxxxxxxxxxxx Fri Feb 3 01:28:55 2023 From: Eric Biggers <ebiggers@xxxxxxxxxx> Date: Thu, 2 Feb 2023 16:27:10 -0800 Subject: exit: Put an upper limit on how often we can oops To: stable@xxxxxxxxxxxxxxx Cc: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, SeongJae Park <sj@xxxxxxxxxx>, Seth Jenkins <sethjenkins@xxxxxxxxxx>, Jann Horn <jannh@xxxxxxxxxx>, "Eric W . Biederman" <ebiederm@xxxxxxxxxxxx>, linux-hardening@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Luis Chamberlain <mcgrof@xxxxxxxxxx> Message-ID: <20230203002717.49198-9-ebiggers@xxxxxxxxxx> From: Jann Horn <jannh@xxxxxxxxxx> 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@xxxxxxxxxx> Link: https://lore.kernel.org/r/20221107201317.324457-1-jannh@xxxxxxxxxx Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Link: https://lore.kernel.org/r/20221117234328.594699-2-keescook@xxxxxxxxxxxx Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- Documentation/sysctl/kernel.txt | 9 ++++++++ kernel/exit.c | 43 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 52 insertions(+) --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -51,6 +51,7 @@ show up in /proc/sys/kernel: - msgmnb - msgmni - nmi_watchdog +- oops_limit - osrelease - ostype - overflowgid @@ -555,6 +556,14 @@ 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: # cat osrelease --- a/kernel/exit.c +++ b/kernel/exit.c @@ -68,6 +68,33 @@ #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--; @@ -924,10 +951,26 @@ 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); } Patches currently in stable-queue which might be from stable-owner@xxxxxxxxxxxxxxx are queue-4.19/panic-unset-panic_on_warn-inside-panic.patch queue-4.19/objtool-add-a-missing-comma-to-avoid-string-concatenation.patch queue-4.19/hexagon-fix-function-name-in-die.patch queue-4.19/exit-add-and-use-make_task_dead.patch queue-4.19/h8300-fix-build-errors-from-do_exit-to-make_task_dead-transition.patch queue-4.19/panic-consolidate-open-coded-panic_on_warn-checks.patch queue-4.19/exit-put-an-upper-limit-on-how-often-we-can-oops.patch queue-4.19/panic-introduce-warn_limit.patch queue-4.19/exit-allow-oops_limit-to-be-disabled.patch queue-4.19/ia64-make-ia64_mca_recovery-bool-instead-of-tristate.patch queue-4.19/exit-use-read_once-for-all-oops-warn-limit-reads.patch queue-4.19/exit-expose-oops_count-to-sysfs.patch queue-4.19/panic-expose-warn_count-to-sysfs.patch queue-4.19/docs-fix-path-paste-o-for-sys-kernel-warn_count.patch queue-4.19/sysctl-add-a-new-register_sysctl_init-interface.patch