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Date: Wed, 18 Mar 2015 14:55:56 -0700 From: Andy Lutomirski <luto@...capital.net> To: Denys Vlasenko <dvlasenk@...hat.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Stefan Seyfried <stefan.seyfried@...glemail.com>, Takashi Iwai <tiwai@...e.de>, X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org> Subject: Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related? On Wed, Mar 18, 2015 at 2:42 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote: > On 03/18/2015 10:32 PM, Linus Torvalds wrote: >> On Wed, Mar 18, 2015 at 12:26 PM, Andy Lutomirski <luto@...capital.net> wrote: >>>> >>>> crash> disassemble page_fault >>>> Dump of assembler code for function page_fault: >>>> 0xffffffff816834a0 <+0>: data32 xchg %ax,%ax >>>> 0xffffffff816834a3 <+3>: data32 xchg %ax,%ax >>>> 0xffffffff816834a6 <+6>: data32 xchg %ax,%ax >>>> 0xffffffff816834a9 <+9>: sub $0x78,%rsp >>>> 0xffffffff816834ad <+13>: callq 0xffffffff81683620 <error_entry> >>> >>> The callq was the double-faulting instruction, and it is indeed the >>> first function in here that would have accessed the stack. (The sub >>> *changes* rsp but isn't a memory access.) So, since RSP is bogus, we >>> page fault, and the page fault is promoted to a double fault. The >>> surprising thing is that the page fault itself seems to have been >>> delivered okay, and RSP wasn't on a page boundary. >> >> Not at all surprising, and sure it was on a page boundry.. >> >> Look closer. >> >> %rsp is 00007fffa55eafb8. >> >> But that's *after* page_fault has done that >> >> sub $0x78,%rsp >> >> so %rsp when the page fault happened was 0x7fffa55eb030. Which is a >> different page. Ah, I forgot to add 0x78. You're right, of course. >> >> And that page happened to be mapped. >> >> So what happened is: >> >> - we somehow entered kernel mode without switching stacks >> >> (ie presumably syscall) >> >> - the user stack was still fine >> >> - we took a page fault, which once again didn't switch stacks, >> because we were already in kernel mode. And this page fault worked, >> because it just pushed the error code onto the user stack which was >> mapped. >> >> - we now took a second page fault within the page fault handler, >> because now the stack pointer has been decremented and points one user >> page down that is *not* mapped, so now that page fault cannot push the >> error code and return information. >> >> Now, how we took that original page fault is sadly not very clear at >> all. I agree that it's something about system-call (how could we not >> change stacks otherwise), but why it should have started now, I don't >> know. I don't think "system_call" has changed at all. >> >> Maybe there is something wrong with the new "ret_from_sys_call" logic, >> and that "use sysret to return to user mode" thing. Because this code >> sequence: >> >> + movq (RSP-RIP)(%rsp),%rsp >> + USERGS_SYSRET64 >> >> in 'irq_return_via_sysret' is new to 4.0, and instead of entering the >> kernel with a user stack poiinter, maybe we're *exiting* the kernel, >> and have just reloaded the user stack pointer when "USERGS_SYSRET64" >> takes some fault. > > Yes, so far we happily thought that SYSRET never fails... > > This merits adding some code which would at least BUG_ON > if the faulting address is seen to match SYSRET64. sysret64 can only fail with #GP, and we're totally screwed if that happens, although I agree about the BUG_ON in principle. Where would we add it that would help in this case, though? We never even made it to C code. In any event, this was a page fault. sysret64 doesn't access memory. > > Now we only check for faulting IRETQ: > > error_kernelspace: > CFI_REL_OFFSET rcx, RCX+8 > incl %ebx > leaq native_irq_return_iret(%rip),%rcx > cmpq %rcx,RIP+8(%rsp) > je error_bad_iret > >> >> Is PARAVIRT enabled? The three nop's at the beginning of 'page_fault' >> makes me suspect it is, and that that is some paravirt rewriting >> area. What does paravirt go for that USERGS_SYSRET64 (or for >> SWAPGS_UNSAFE_STACK, for that matter). On Xen, it goes to xen_sysret64, which touches the same percpu variables that we touch on entry. So I still like my percpu vmap fault hypothesis, even though I don't understand what would trigger it. At the risk of asking awful questions, what happens if we deliver an IST interrupt in vmx_handle_external_intr? Can that happen? It can't be a good thing if it happens. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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