CVE-2024-50164

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 checkmemsize_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 MEMUNINIT's initial meaning that "the passed buffer to the BPF helper does not need to be initialized" which was added back in commit 435faee1aae9 ("bpf, verifier: add ARGPTRTORAW_STACK type") got overloaded over time with "the passed buffer is being written to".

The problem however is that checks such as the above which were added later via 06c1c049721a ("bpf: 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 MEMUNINIT back to its original meaning, and having MEMWRITE as an annotation to BPF helpers in order to then trigger the BPF verifier checks for writing to memory.

Some notes: checkargpairok() ensures that for ARGCONSTSIZE{,ORZERO} we can access fn->argtype[arg - 1] since it must contain a preceding ARGPTRTOMEM. For checkmemreg() the meta argument can be removed altogether since we do check both BPFREAD and BPFWRITE. Same for the equivalent checkkfuncmemsize_reg().