CVE-2022-49236

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix UAF due to race between btf_try_get_module and load_module

While working on code to populate kfunc BTF ID sets for module BTF from its initcall, I noticed that by the time the initcall is invoked, the module BTF can already be seen by userspace (and the BPF verifier). The existing btf_try_get_module calls try_module_get which only fails if mod->state == MODULE_STATE_GOING, i.e. it can increment module reference when module initcall is happening in parallel.

Currently, BTF parsing happens from MODULE_STATE_COMING notifier callback. At this point, the module initcalls have not been invoked. The notifier callback parses and prepares the module BTF, allocates an ID, which publishes it to userspace, and then adds it to the btf_modules list allowing the kernel to invoke btf_try_get_module for the BTF.

However, at this point, the module has not been fully initialized (i.e. its initcalls have not finished). The code in module.c can still fail and free the module, without caring for other users. However, nothing stops btf_try_get_module from succeeding between the state transition from MODULE_STATE_COMING to MODULE_STATE_LIVE.

This leads to a use-after-free issue when BPF program loads successfully in the state transition, load_modules do_init_module call fails and frees the module, and BPF program fd on close calls module_put for the freed module. Future patch has test case to verify we dont regress in this area in future.

There are multiple points after prepare_coming_module (in load_module) where failure can occur and module loading can return error. We illustrate and test for the race using the last point where it can practically occur (in module __init function).

An illustration of the race:

CPU 0 CPU 1 load_module notifier_call(MODULE_STATE_COMING) btf_parse_module btf_alloc_id // Published to userspace list_add(&btf_mod->list, btf_modules) mod->init(…) … ^ bpf_check | check_pseudo_btf_id | btf_try_get_module | returns true | … … | module __init in progress return prog_fd | … … V if (ret < 0) free_module(mod) … close(prog_fd) … bpf_prog_free_deferred module_put(used_btf.mod) // use-after-free

We fix this issue by setting a flag BTF_MODULE_F_LIVE, from the notifier callback when MODULE_STATE_LIVE state is reached for the module, so that we return NULL from btf_try_get_module for modules that are not fully formed. Since try_module_get already checks that module is not in MODULE_STATE_GOING state, and that is the only transition a live module can make before being removed from btf_modules list, this is enough to close the race and prevent the bug.

A later selftest patch crafts the race condition artifically to verify that it has been fixed, and that verifier fails to load program (with ENXIO).

Lastly, a couple of comments:

1. Even if this race didnt exist, it seems more appropriate to only access resources (ksyms and kfuncs) of a fully formed module which has been initialized completely.

2. This patch was born out of need for synchronization against module initcall for the next patch, so it is needed for correctness even without the aforementioned race condition. The BTF resources initialized by module initcall are set up once and then only looked up, so just waiting until the initcall has finished ensures correct behavior.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0481baa2318cb1ab13277715da6cdbb657807b3f
• https://git.kernel.org/stable/c/18688de203b47e5d8d9d0953385bf30b5949324f
• https://git.kernel.org/stable/c/51b82141fffa454abf937a8ff0b8af89e4fd0c8f
• https://git.kernel.org/stable/c/d7fccf264b1a785525b366a5b7f8113c756187ad