Date: Thu, 10 Sep 2020 16:52:48 +0200 From: Salvatore Bonaccorso <carnil@...ian.org> To: Andy Lutomirski <luto@...nel.org> Cc: oss security list <oss-security@...ts.openwall.com> Subject: Re: CVE Request: Linux kernel vsyscall page refcounting error On Tue, Sep 08, 2020 at 08:33:00AM -0700, Andy Lutomirski wrote: > 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 CVE-2020-25221 has been assigned by MITRE for this issue (note one cannot request anymore CVEs through that list but one can use https://cveform.mitre.org/) Regards, Salvatore
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