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Date: Thu, 10 Sep 2020 16:52:48 +0200
From: Salvatore Bonaccorso <>
To: Andy Lutomirski <>
Cc: oss security list <>
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 <>
> 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 <>
> 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


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