CVE-2024-50164 Information

Description

In the Linux kernel the following vulnerability has been resolved:

bpf: Fix overloading of MEM_UNINIT’s meaning

Lonial reported an issue in the BPF verifier where check_mem_size_reg() has the following code:

if (!tnum_is_const(reg->var_off))
    / For unprivileged variable accesses disable raw
      mode so that the program is required to
      initialize all the memory that the helper could
      just partially fill up.
     /
     meta = NULL;

This means that writes are not checked when the register containing the size of the passed buffer has not a fixed size. Through this bug a BPF program can write to a map which is marked as read-only for example .rodata global maps.

The problem is that MEM_UNINIT’s initial meaning that he passed buffer to the BPF helper does not need to be initialized\ which was added back in commit 435faee1aae9 (pf verifier: add ARG_PTR_TO_RAW_STACK type) got overloaded over time with he passed buffer is being written to.

The problem however is that checks such as the above which were added later via 06c1c049721a (pf: allow helpers access to variable memory) set meta to NULL in order force the user to always initialize the passed buffer to the helper. Due to the current double meaning of MEM_UNINIT this bypasses verifier write checks to the memory (not boundary checks though) and only assumes the latter memory is read instead.

Fix this by reverting MEM_UNINIT back to its original meaning and having MEM_WRITE as an annotation to BPF helpers in order to then trigger the BPF verifier checks for writing to memory.

Some notes: check_arg_pair_ok() ensures that for ARG_CONST_SIZE_OR_ZERO we can access fn->arg_type[arg - 1] since it must contain a preceding ARG_PTR_TO_MEM. For check_mem_reg() the meta argument can be removed altogether since we do check both BPF_READ and BPF_WRITE. Same for the equivalent check_kfunc_mem_size_reg().

Reference

https://git.kernel.org/stable/c/48068ccaea957469f1adf78dfd2c1c9a7e18f0fe https://git.kernel.org/stable/c/54bc31682660810af1bed7ca7a19f182df8d3df8 https://git.kernel.org/stable/c/8ea607330a39184f51737c6ae706db7fdca7628e