Date: Tue, 8 Sep 2020 08:33:00 -0700 From: Andy Lutomirski <luto@...nel.org> To: oss security list <oss-security@...ts.openwall.com> Subject: CVE Request: Linux kernel vsyscall page refcounting error Linux 5.7 and 5.8 have a bug in the reference counting of the struct page that backs the vsyscall page. The result is a refcount underflow. This can be triggered by any 64-bit process that is permitted to use ptrace() or process_vm_readv(). A creative attacker can probably achieve kernel code escalation by using this bug. You can prevent the issue from triggering by booting with vsyscall=xonly or vsyscall=none. You can also effectively hotpatch a kernel with suitable hardening options by running the updated test case noted below -- the test case will underflow the refcount past zero, preventing further use of the page. (A real attacker would carefully underflow it exactly to zero but not past.) Or you can fix your kernel. (No one should be using vsyscall=emulate any more unless they have a very specific use case that requires it. vsyscall=xonly is better in almost all cases. For some reason, Fedora still seems to be using emulate mode, though.) Fixed by: commit 9fa2dd946743ae6f30dc4830da19147bf100a7f2 Author: Dave Hansen <dave.hansen@...ux.intel.com> Date: Thu Sep 3 13:40:28 2020 -0700 mm: fix pin vs. gup mismatch with gate pages and tested a little better by: commit 8891adc61dce2a8a41fc0c23262b681c3ec4b73a Author: Andy Lutomirski <luto@...nel.org> Date: Thu Sep 3 13:40:30 2020 -0700 selftests/x86/test_vsyscall: Improve the process_vm_readv() test
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