| From bippy-5f407fcff5a0 Mon Sep 17 00:00:00 2001 |
| From: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| To: <linux-cve-announce@vger.kernel.org> |
| Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org> |
| Subject: CVE-2021-47608: bpf: Fix kernel address leakage in atomic fetch |
| |
| Description |
| =========== |
| |
| 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 didn't 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 we're 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. |
| |
| The Linux kernel CVE team has assigned CVE-2021-47608 to this issue. |
| |
| |
| Affected and fixed versions |
| =========================== |
| |
| Issue introduced in 5.12 with commit 37086bfdc737ea6f66bf68dcf16757004d68e1e1 and fixed in 5.15.11 with commit 423628125a484538111c2c6d9bb1588eb086053b |
| Issue introduced in 5.12 with commit 37086bfdc737ea6f66bf68dcf16757004d68e1e1 and fixed in 5.16 with commit 7d3baf0afa3aa9102d6a521a8e4c41888bb79882 |
| |
| Please see https://www.kernel.org for a full list of currently supported |
| kernel versions by the kernel community. |
| |
| Unaffected versions might change over time as fixes are backported to |
| older supported kernel versions. The official CVE entry at |
| https://cve.org/CVERecord/?id=CVE-2021-47608 |
| will be updated if fixes are backported, please check that for the most |
| up to date information about this issue. |
| |
| |
| Affected files |
| ============== |
| |
| The file(s) affected by this issue are: |
| kernel/bpf/verifier.c |
| |
| |
| Mitigation |
| ========== |
| |
| The Linux kernel CVE team recommends that you update to the latest |
| stable kernel version for this, and many other bugfixes. Individual |
| changes are never tested alone, but rather are part of a larger kernel |
| release. Cherry-picking individual commits is not recommended or |
| supported by the Linux kernel community at all. If however, updating to |
| the latest release is impossible, the individual changes to resolve this |
| issue can be found at these commits: |
| https://git.kernel.org/stable/c/423628125a484538111c2c6d9bb1588eb086053b |
| https://git.kernel.org/stable/c/7d3baf0afa3aa9102d6a521a8e4c41888bb79882 |
