From de586785b981d24ccc432becd1cdb55c81bb09fa Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Tue, 24 Jan 2023 11:29:57 -0800 Subject: 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 Link: https://lore.kernel.org/r/20221107201317.324457-1-jannh@google.com Reviewed-by: Luis Chamberlain Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20221117234328.594699-2-keescook@chromium.org Signed-off-by: Eric Biggers Signed-off-by: Sasha Levin --- Documentation/admin-guide/sysctl/kernel.rst | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'Documentation') diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index a4b1ebc2e70b..cd9247b48fc7 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -663,6 +663,14 @@ 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 =========================== -- cgit v1.2.3