CVE-2024-49861

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

bpf: Fix helper writes to read-only maps

Lonial found an issue that despite user- and BPF-side frozen BPF map (like in case of .rodata), it was still possible to write into it from a BPF program side through specific helpers having ARG_PTR_TO_{LONG,INT} as arguments.

In check_func_arg() when the argument is as mentioned, the meta->raw_mode is never set. Later, check_helper_mem_access(), under the case of PTR_TO_MAP_VALUE as register base type, it assumes BPF_READ for the subsequent call to check_map_access_type() and given the BPF map is read-only it succeeds.

The helpers really need to be annotated as ARG_PTR_TO_{LONG,INT} | MEM_UNINIT when results are written into them as opposed to read out of them. The latter indicates that its okay to pass a pointer to uninitialized memory as the memory is written to anyway.

However, ARG_PTR_TO_{LONG,INT} is a special case of ARG_PTR_TO_FIXED_SIZE_MEM just with additional alignment requirement. So it is better to just get rid of the ARG_PTR_TO_{LONG,INT} special cases altogether and reuse the fixed size memory types. For this, add MEM_ALIGNED to additionally ensure alignment given these helpers write directly into the args via = val. The .arg_size has been initialized reflecting the actual sizeof(*).

MEM_ALIGNED can only be used in combination with MEM_FIXED_SIZE annotated argument types, since in !MEM_FIXED_SIZE cases the verifier does not know the buffer size a priori and therefore cannot blindly write * = val.

Affected Software

References

• https://git.kernel.org/stable/c/1e75d25133158b525e0456876e9bcfd6b2993fd5
• https://git.kernel.org/stable/c/2ed98ee02d1e08afee88f54baec39ea78dc8a23c
• https://git.kernel.org/stable/c/32556ce93bc45c730829083cb60f95a2728ea48b
• https://git.kernel.org/stable/c/a2c8dc7e21803257e762b0bf067fd13e9c995da0