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Date: Tue, 3 Feb 2015 10:21:58 +0300
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Re: Linux kernel: multiple x86_64 vulnerabilities

On Mon, Dec 15, 2014 at 10:01:19AM -0800, Andy Lutomirski wrote:
> CVE-2014-9322: local privilege escalation, all kernel versions

Here's Rafal Wojtczuk's writeup on exploiting it:

http://labs.bromium.com/2015/02/02/exploiting-badiret-vulnerability-cve-2014-9322-linux-kernel-privilege-escalation/

It's been a while since Andy posted this, so I'll quote the rest of his
message for context:

> Any kernel that is not patched against CVE-2014-9090 is vulnerable to
> privilege escalation due to incorrect handling of a #SS fault caused
> by an IRET instruction.  In particular, if IRET executes on a
> writeable kernel stack (this was always the case before 3.16 and is
> sometimes the case on 3.16 and newer), the assembly function
> general_protection will execute with the user's gsbase and the
> kernel's gsbase swapped.
> 
> This is likely to be easy to exploit for privilege escalation, except
> on systems with SMAP or UDEREF.  On those systems, assuming that the
> mitigation works correctly, the impact of this bug may be limited to
> massive memory corruption and an eventual crash or reboot.
> 
> As with CVE-2014-9090, this is fixed by:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86/kernel/entry_64.S?id=6f442be2fb22be02cafa606f1769fa1e6f894441
> 
> The related fix to remove bad_iret is also an effective mitigation to
> prevent a bug like this from being reintroduced:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86/kernel/entry_64.S?id=b645af2d5905c4e32399005b867987919cbfc3ae
> 
> Partial credit for this bug goes to Borislav Petkov, who asked pointed
> questions about CVE-2014-9090, causing me to realize that there were
> two separate bugs in #SS handling.  The first bug (CVE-2014-9090)
> caused a fatal double fault, masking the second bug that caused the
> gsbase issue.
> 
> ----------
> 
> The next two bugs are related to espfix.  The IRET instruction has IMO
> a blatant design flaw: IRET to a 16-bit user stack segment will leak
> bits 31:16 of the kernel stack pointer.  This flaw exists on 32-bit
> and 64-bit systems.  32-bit Linux kernels have mitigated this leak for
> a long time, and 64-bit Linux kernels have mitigated this leak since
> 3.16.  The mitigation is called espfix.
> 
> CVE-2014-8133: espfix bypass using set_thread_area
> 
> On all kernels, a valid 16-bit stack segment can be created using
> set_thread_area.  Arranging to return to such a stack segment will
> bypass espfix, leaking bits 31:16 of the kernel stack pointer.  Fixed
> by:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86?id=41bdc78544b8a93a9c6814b8bbbfef966272abbe
> 
> CVE-2014-8134: espfix was broken on 32-bit KVM paravirt guests
> 
> espfix was completely broken on 32-bit Linux KVM guests with
> CONFIG_KVM_GUEST=y.  Fixed by:
> 
> https://git.kernel.org/cgit/virt/kvm/kvm.git/commit/?h=linux-next&id=29fa6825463c97e5157284db80107d1bfac5d77b
> 
> This commit hasn't made it to Linus' tree yet.
> 
> ----------
> 
> CVE-2014-9090 (previously announced), CVE-2014-9322, CVE-2014-8133,
> and CVE-2014-8134 can be tested by sigreturn_32, available here:
> 
> https://gitorious.org/linux-test-utils/linux-clock-tests/source/10b9a7d317f6d8ae5f32bcb4bbbb186acdd6b89a
> 
> Save your data before running this on a production system.  If you a
> vulnerable to CVE-2014-9090 or CVE-2014-9322, the test will crash your
> system.  The espfix issues will cause warnings and failures that
> mention register mismatches.
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC

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