Date:	Thu, 3 Oct 2013 09:47:18 -0400 (EDT)
From:	Dave Anderson <anderson@...hat.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
Cc:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	kernel-hardening@...ts.openwall.com,
	Aaron Durbin <adurbin@...gle.com>,
	Eric Northup <digitaleric@...gle.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...gle.com>,
	Mathias Krause <minipli@...glemail.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic



----- Original Message -----
> (2013/10/02 18:13), HATAYAMA Daisuke wrote:
> > (2013/10/02 16:48), Kees Cook wrote:
> <cut>
> >>>> +
> >>>> +     return 0;
> >>>> +}
> >>>> +
> >>>> +/*
> >>>>     * Determine if we were loaded by an EFI loader.  If so, then we have also been
> >>>>     * passed the efi memmap, systab, etc., so we should use these data structures
> >>>>     * for initialization.  Note, the efi init code path is determined by the
> >>>> @@ -1242,3 +1256,15 @@ void __init i386_reserve_resources(void)
> >>>>    }
> >>>>
> >>>>    #endif /* CONFIG_X86_32 */
> >>>> +
> >>>> +static struct notifier_block kernel_offset_notifier = {
> >>>> +     .notifier_call = dump_kernel_offset
> >>>> +};
> >>>> +
> >>>> +static int __init register_kernel_offset_dumper(void)
> >>>> +{
> >>>> +     atomic_notifier_chain_register(&panic_notifier_list,
> >>>> +                                     &kernel_offset_notifier);
> >>>> +     return 0;
> >>>> +}
> >>>> +__initcall(register_kernel_offset_dumper);
> >>>>
> >>>
> >>> Panic notifier is not executed if kdump is enabled. Maybe, Chrome OS doesn't use
> >>> kdump? Anyway, kdump related tools now calculate phys_base from memory map
> >>> information passed as ELF PT_LOAD entries like below.
> >>
> >> Correct, we are not currently using kdump.
> >>
> >>> $ LANG=C readelf -l vmcore-rhel6up4
> >>>
> >>> Elf file type is CORE (Core file)
> >>> Entry point 0x0
> >>> There are 5 program headers, starting at offset 64
> >>>
> >>> Program Headers:
> >>>    Type           Offset             VirtAddr           PhysAddr
> >>>                   FileSiz            MemSiz              Flags  Align
> >>>    NOTE           0x0000000000000158 0x0000000000000000  0x0000000000000000
> >>>                   0x0000000000000b08 0x0000000000000b08         0
> >>>    LOAD           0x0000000000000c60 0xffffffff81000000  0x0000000001000000
> >>>                   0x000000000103b000 0x000000000103b000  RWE    0
> >>>    LOAD           0x000000000103bc60 0xffff880000001000  0x0000000000001000
> >>>                   0x000000000009cc00 0x000000000009cc00  RWE    0
> >>>    LOAD           0x00000000010d8860 0xffff880000100000  0x0000000000100000
> >>>                   0x0000000002f00000 0x0000000002f00000  RWE    0
> >>>    LOAD           0x0000000003fd8860 0xffff880013000000  0x0000000013000000
> >>>                   0x000000002cffd000 0x000000002cffd000  RWE    0
> >>>
> >>> Each PT_LOAD entry is assigned to virtual and physical address. In this case,
> >>> 1st PT_LOAD entry belongs to kernel text mapping region, from which we can
> >>> calculate phys_base value.
> >>
> >> It seems like all the information you need would still be available?
> >> The virtual address is there, so it should be trivial to see the
> >> offset, IIUC.
> >>
> >
> > Partially yes. I think OK to analyze crash dump by crash utility, a gdb-based
> > symbolic debugger for kernel, since phys_base absorbs kernel offset caused by
> > relocation and phys_base is available in the way I explained above.
> >
> > However, the gained phys_base is not correct one, exactly phys_base + offset_by_relocation.
> > When analyzing crash dump by crash utility, we use debug information generated
> > during kernel build, which we install as kernel-debuginfo on RHEL for example.
> > Symbols in debuginfo have statically assigned addresses at build so we see
> > the statically assigned addresses during debugging and we see
> > phys_base + offset_by_relocation as phys_base. This would be problematic
> > if failure on crash dump is relevant to the relocated addresses, though I don't
> > immediately come up with crash senario where relocated symbol is defitely necessary.
> >
> > Still we can get relocated addresses if kallsyms is enabled on the kernel,
> > but kallsyms and relocatable kernels are authogonal. I don't think it natural
> > to rely on kallsyms. It seems natural to export relocation information newly
> > as debugging information.
> >
> 
> I was confused yesterday. As I said above, kdump related tools now don't support
> relocation on x86_64, phys_base only. kdump related tools think of present kernel
> offset as phys_base. Then, they reflect kernel offset caused by relocation in
> physical addresses only, not in virtual addresses. This obviously affects the
> tools.
> 
> BTW, relocation looks more sophisticated than phys_base one. Is it possible to
> switch from phys_base one to relocation on x86_64? On x86, relocation is used so
> I guess x86_64 can work in the same way. Is there something missing?
> Is there what phys_base can but relocation cannot on x86_64?
> 
> And, Dave, is there feature for crash utility to treat relocation now?

Well sort of, there are couple guessing-game kludges that can be used.

For 32-bit x86 systems configured with a CONFIG_PHYSICAL_START value 
that is larger than its CONFIG_PHYSICAL_ALIGN value, such that the 
vmlinux symbol values do not match their relocated virtual address
values, there are two options for analyzing dumpfiles:

(1) there is a "--reloc size" command line option, presuming that
    you know what it is.
(2) take a snapshot of the /proc/kallsyms file from the crashing
    system into a file, and put it on the command line, similar
    to putting a System.map file on the command line in order to
    override the symbol values in the vmlinux file.

In those cases, we have to alter all of the symbols seen in the
vmlinux file, and go into a backdoor into the embedded gdb module 
to patch/modify the symbol values.

On live x86 systems, the two options above are not necessary if
/proc/kallsyms exists, because its contents can be checked against
the vmlinux file symbol values, and the relocation calculated.

For x86_64, the --reloc argument has never been needed.  But if
for whatever reason the "phys_base" value cannot be determined,
it can be forced with the "--machdep phys_base=addr" option, 
again presuming you know what it is.   

Dave

> --
> Thanks.
> HATAYAMA, Daisuke
> 
> 
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