CVE-2024-56702 Information

Description

In the Linux kernel the following vulnerability has been resolved:

bpf: Mark raw_tp arguments with PTR_MAYBE_NULL

Arguments to a raw tracepoint are tagged as trusted which carries the semantics that the pointer will be non-NULL. However in certain cases a raw tracepoint argument may end up being NULL. More context about this issue is available in 0.

Thus there is a discrepancy between the reality that raw_tp arguments can actually be NULL and the verifier’s knowledge that they are never NULL causing explicit NULL checks to be deleted and accesses to such pointers potentially crashing the kernel.

To fix this mark raw_tp arguments as PTR_MAYBE_NULL and then special case the dereference and pointer arithmetic to permit it and allow passing them into helpers/kfuncs; these exceptions are made for raw_tp programs only. Ensure that we don’t do this when ref_obj_id > 0 as in that case this is an acquired object and doesn’t need such adjustment.

The reason we do mask_raw_tp_trusted_reg logic is because other will recheck in places whether the register is a trusted_reg and then consider our register as untrusted when detecting the presence of the PTR_MAYBE_NULL flag.

To allow safe dereference we enable PROBE_MEM marking when we see loads into trusted pointers with PTR_MAYBE_NULL.

While trusted raw_tp arguments can also be passed into helpers or kfuncs where such broken assumption may cause issues a future patch set will tackle their case separately as PTR_TO_BTF_ID (without PTR_TRUSTED) can already be passed into helpers and causes similar problems. Thus they are left alone for now.

It is possible that these checks also permit passing non-raw_tp args that are trusted PTR_TO_BTF_ID with null marking. In such a case allowing dereference when pointer is NULL expands allowed behavior so won’t regress existing programs and the case of passing these into helpers is the same as above and will be dealt with later.

Also update the failure case in tp_btf_nullable selftest to capture the new behavior as the verifier will no longer cause an error when directly dereference a raw tracepoint argument marked as __nullable.

Reference

https://git.kernel.org/stable/c/3634d4a310820567fc634bf8f1ee2b91378773e8 https://git.kernel.org/stable/c/c9b91d2d54175f781ad2c361cb2ac2c0e29b14b6 https://git.kernel.org/stable/c/cb4158ce8ec8a5bb528cc1693356a5eb8058094d