CVE-2021-47608

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

bpf: Fix kernel address leakage in atomic fetch

The change in commit 37086bfdc737 (bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH) around check_mem_access() handling is buggy since this would allow for unprivileged users to leak kernel pointers. For example, an atomic fetch/and with -1 on a stack destination which holds a spilled pointer will migrate the spilled register type into a scalar, which can then be exported out of the program (since scalar != pointer) by dumping it into a map value.

The original implementation of XADD was preventing this situation by using a double call to check_mem_access() one with BPF_READ and a subsequent one with BPF_WRITE, in both cases passing -1 as a placeholder value instead of register as per XADD semantics since it didnt contain a value fetch. The BPF_READ also included a check in check_stack_read_fixed_off() which rejects the program if the stack slot is of __is_pointer_value() if dst_regno < 0. The latter is to distinguish whether were dealing with a regular stack spill/ fill or some arithmetical operation which is disallowed on non-scalars, see also 6e7e63cbb023 (bpf: Forbid XADD on spilled pointers for unprivileged users) for more context on check_mem_access() and its handling of placeholder value -1.

One minimally intrusive option to fix the leak is for the BPF_FETCH case to initially check the BPF_READ case via check_mem_access() with -1 as register, followed by the actual load case with non-negative load_reg to propagate stack bounds to registers.

Affected Software

References

• https://git.kernel.org/stable/c/423628125a484538111c2c6d9bb1588eb086053b
• https://git.kernel.org/stable/c/7d3baf0afa3aa9102d6a521a8e4c41888bb79882