Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

[HATAYAMA Daisuke]

(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?

--
Thanks.
HATAYAMA, Daisuke

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