bpf: Harden bloom filter sizing and indexing on 32-bit kernels

Overview

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

bpf: Harden bloom filter sizing and indexing on 32-bit kernels

bloom_map_alloc() has two 32-bit-specific problems when the computed bitmap reaches the U32_MAX fallback case.

First, BITS_TO_BYTES(U32_MAX) is evaluated with 32-bit arithmetic. The addition performed by DIV_ROUND_UP wraps, so the map allocates only the fixed-size bloom filter object while keeping bitset_mask == U32_MAX. Subsequent updates can then write past the allocated object.

Second, fixing only the allocation size is not sufficient. The bloom hash is a u32, but set_bit() takes a signed long bit number and x86 test_bit() eventually feeds the index to variable_test_bit(long, ...). On 32-bit kernels, hashes in [0x80000000, U32_MAX] therefore become negative bit offsets. x86 bt/bts with a memory operand interpret those offsets relative to the supplied base, so a map with bitset_mask == U32_MAX can read or write before bloom->bitset even after allocating the full 512 MiB bitmap.

Keep the U32_MAX fallback, but split each hash into a word pointer and an in-word bit number before calling test_bit() or set_bit(). The bitops argument is then always in [0, BITS_PER_LONG - 1], while BIT_WORD(h) still selects the intended word in the full bitmap.

Compute the bitset size from (u64)bitset_mask + 1 before passing the final size to bpf_map_area_alloc(). This fixes the original under-allocation and keeps the allocated storage consistent with the addressable bitset.

Exploitation note: local privilege escalation is possible on a 32-bit x86 kernel using the under-allocation bug from a binary with CAP_BPF.

Affected

• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < a6183bc683f97f4317f7e84939ca7fff37c5688b
• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < 80551bf8912c42d1e3d55eec6fa3c40f306c3de8
• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < 3b7a13eccfcf97714ffc1ca6aa663d66d4a30e29
• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < 272fcb4ba6fab678db0eb966dc81c4c804bef64a
• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < dff481e12b3f127739f6a4ea7cef2c25dc12e056
• Linux >= 9330986c03006ab1d33d243b7cfe598a7a3c1baa < 11c1e836710dcba03e50454a4eedfdbaf8d3050e
• Linux 5.16

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9 · updated 2026-09-27 · vex