In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix percpu map update indexing with sparse CPU IDs
Per-CPU array, hash, and cgroup storage map updates without BPF_F_CPU or BPF_F_ALL_CPUS use a value buffer whose per-CPU slots are packed in possible-CPU order. The buffer is sized as:
round_up(value_size, 8) * num_possible_cpus()
The update paths iterate over possible CPUs, but use the logical CPU ID to calculate the source offset:
value + size * cpu
This only works when possible CPU IDs are contiguous starting at zero.
For example, with a possible CPU mask of 0,2-3, the buffer contains three slots corresponding to CPUs 0, 2, and 3. CPU2 is therefore expected to use slot 1 and CPU3 slot 2. Instead, the current code uses slots 2 and 3 respectively, causing incorrect per-CPU values and an out-of-bounds read from the update buffer for CPU3.
The corresponding lookup paths already use a dense offset while iterating over possible CPUs. Do the same for the array, hash, and cgroup storage update paths, advancing the source offset once for each possible CPU. BPF_F_ALL_CPUS continues to use the same value for every CPU.