| 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-2025-21632: x86/fpu: Ensure shadow stack is active before "getting" registers |
| |
| Description |
| =========== |
| |
| In the Linux kernel, the following vulnerability has been resolved: |
| |
| x86/fpu: Ensure shadow stack is active before "getting" registers |
| |
| The x86 shadow stack support has its own set of registers. Those registers |
| are XSAVE-managed, but they are "supervisor state components" which means |
| that userspace can not touch them with XSAVE/XRSTOR. It also means that |
| they are not accessible from the existing ptrace ABI for XSAVE state. |
| Thus, there is a new ptrace get/set interface for it. |
| |
| The regset code that ptrace uses provides an ->active() handler in |
| addition to the get/set ones. For shadow stack this ->active() handler |
| verifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the |
| thread struct. The ->active() handler is checked from some call sites of |
| the regset get/set handlers, but not the ptrace ones. This was not |
| understood when shadow stack support was put in place. |
| |
| As a result, both the set/get handlers can be called with |
| XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to |
| return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an |
| ssp_active() check to avoid surprising the kernel with shadow stack |
| behavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That |
| check just happened to avoid the warning. |
| |
| But the ->get() side wasn't so lucky. It can be called with shadow stacks |
| disabled, triggering the warning in practice, as reported by Christina |
| Schimpe: |
| |
| WARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0 |
| [...] |
| Call Trace: |
| <TASK> |
| ? show_regs+0x6e/0x80 |
| ? ssp_get+0x89/0xa0 |
| ? __warn+0x91/0x150 |
| ? ssp_get+0x89/0xa0 |
| ? report_bug+0x19d/0x1b0 |
| ? handle_bug+0x46/0x80 |
| ? exc_invalid_op+0x1d/0x80 |
| ? asm_exc_invalid_op+0x1f/0x30 |
| ? __pfx_ssp_get+0x10/0x10 |
| ? ssp_get+0x89/0xa0 |
| ? ssp_get+0x52/0xa0 |
| __regset_get+0xad/0xf0 |
| copy_regset_to_user+0x52/0xc0 |
| ptrace_regset+0x119/0x140 |
| ptrace_request+0x13c/0x850 |
| ? wait_task_inactive+0x142/0x1d0 |
| ? do_syscall_64+0x6d/0x90 |
| arch_ptrace+0x102/0x300 |
| [...] |
| |
| Ensure that shadow stacks are active in a thread before looking them up |
| in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are |
| set at the same time, the active check ensures that there will be |
| something to find in the XSAVE buffer. |
| |
| [ dhansen: changelog/subject tweaks ] |
| |
| The Linux kernel CVE team has assigned CVE-2025-21632 to this issue. |
| |
| |
| Affected and fixed versions |
| =========================== |
| |
| Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.6.72 with commit 0a3a872214188e4268d31581ed0cd44508e038cf |
| Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.12.10 with commit 6bfe1fc22f462bec87422cdcbec4d7a2f43ff01d |
| Issue introduced in 6.6 with commit 2fab02b25ae7cf5f714ab456b03d9a3fe5ae98c9 and fixed in 6.13 with commit a9d9c33132d49329ada647e4514d210d15e31d81 |
| |
| 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-2025-21632 |
| 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: |
| arch/x86/kernel/fpu/regset.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/0a3a872214188e4268d31581ed0cd44508e038cf |
| https://git.kernel.org/stable/c/6bfe1fc22f462bec87422cdcbec4d7a2f43ff01d |
| https://git.kernel.org/stable/c/a9d9c33132d49329ada647e4514d210d15e31d81 |
