Date: Thu, 23 Jul 2015 16:00:15 +0200 From: Petr Matousek <pmatouse@...hat.com> To: Andy Lutomirski <luto@...capital.net> Cc: oss security list <oss-security@...ts.openwall.com> Subject: Re: Linux x86_64 NMI security issues On Thu, Jul 23, 2015 at 06:34:50AM -0700, Andy Lutomirski wrote: > On Jul 23, 2015 6:28 AM, "Petr Matousek" <pmatouse@...hat.com> wrote: > > On Wed, Jul 22, 2015 at 11:12:00AM -0700, Andy Lutomirski wrote: > > > On kernels that are patched for BadIRET and have a fixup_bad_iret > > > function (which should be most kernels that are keeping up with > > > low-level security issues), there are two cases. > > > > > > Case 1a (more up-to-date kernels where INTERRUPT_RETURN is "jmp > > > irq_return"): fixup_bad_iret will be invoked and will attempt to > > > recover. There's a narrow window in which a new NMI will cause > > > corruption, in which case all bets are off. That could hang, crash, > > > or possibly be exploited for privilege escalation. > > > > > > Case 1b (less up-to-date kernels where INTERRUPT_RETURN is "iretq"): > > > The kernel will try to OOPS due to a bad kernel fault, except that the > > > OOPS will be processed with the wrong gsbase. This is basically the > > > BadIRET condition, and is probably exploitable using similar > > > techniques to BadIRET. > > > > Could you please explain the backtrace leading to this? You mean the > > nested nmi return which invokes INTERRUPT_RETURN and in case > > INTERRUPT_RETURN is "iretq", error_kernelspace won't detect that and > > won't fixup the gs? > > I mean the normal (non-nested) NMI return. If we return with iretq, then > the error_bad_iret fixup won't trigger at all because that iretq > instruction has no fixup entry or swapgs special case. Hmm, sorry, my bad. I've been looking at rhel-7 -- I see two ways to exit the NMI handler there -- INTERRUPT_RETURN after the nested_nmi_out label, which should only trigger on nested NMIs and thus be safe, and the second one after the nmi_restore label which does jmp irq_return. And irq_return is: irq_return: INTERRUPT_RETURN _ASM_EXTABLE(irq_return, bad_iret) and error_kernelspace checking for irq_return error_kernelspace: incl %ebx leaq irq_return(%rip),%rcx cmpq %rcx,RIP+8(%rsp) je error_swapgs Now the paravirt iret case is not protected by the error_kernelspace fixup. How much that is a problem is something I'm going to have a look at ... -- Petr Matousek / Red Hat Product Security PGP: 0xC44977CA 8107 AF16 A416 F9AF 18F3 D874 3E78 6F42 C449 77CA
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