On Mon, Sep 19, 2016 at 3:57 PM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
> On Mon, Sep 19, 2016 at 02:51:54PM -0700, Kees Cook wrote:
>> On Mon, Sep 19, 2016 at 2:37 PM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
>> > Okay, I just built x86_64 default defconfig (on ef98de028afd, half way
>> > through the mm patches on linux-next from the other day where metag
>> > stopped booting). Perhaps I'm missing some important config option to
>> > enable the memory protection (if so I appologise).
>> >
>> > For metag:
>> >
>> > $ readelf -S drivers/tty/pty.o
>> > [Nr] Name Type Addr Off Size ES Flg Lk Inf Al
>> > [51] .data..ro_after_i PROGBITS 00000000 00f0c0 00007c 00 WA 0 0 4
>> >
>> > $ readelf -S vmlinux.bust:
>> > [Nr] Name Type Addr Off Size ES Flg Lk Inf Al
>> > [ 4] .rodata PROGBITS 40190000 194000 04c9c8 00 WA 0 0 64
>> >
>> > And x86_64:
>> >
>> > $ readelf -S drivers/tty/pty.o
>> > [Nr] Name Type Address Offset
>> > Size EntSize Flags Link Info Align
>> > [18] .data..ro_after_i PROGBITS 0000000000000000 00001140
>> > 00000000000000f8 0000000000000000 WA 0 0 64
>> >
>> > $ readelf -S vmlinux
>> > [Nr] Name Type Address Offset
>> > Size EntSize Flags Link Info Align
>> > [ 4] .rodata PROGBITS ffffffff81a00000 00c00000
>> > 00000000002663d0 0000000000000000 WA 0 0 4096
>> >
>> > Both have WA on that section in the object file and the final vmlinux
>> > ELF too.
>>
>> Hm, very true, I never noticed that. Oddly, the LOAD flags don't pay
>> any attention on x86:
>>
>> $ readelf -l vmlinux
>>
>> Elf file type is EXEC (Executable file)
>> Entry point 0x1000000
>> There are 5 program headers, starting at offset 64
>>
>> Program Headers:
>> Type Offset VirtAddr PhysAddr
>> FileSiz MemSiz Flags Align
>> LOAD 0x0000000000200000 0xffffffff81000000 0x0000000001000000
>> 0x0000000000fdc000 0x0000000000fdc000 R E 200000
>> LOAD 0x0000000001200000 0xffffffff82000000 0x0000000002000000
>> 0x0000000000155000 0x0000000000155000 RW 200000
>> LOAD 0x0000000001400000 0x0000000000000000 0x0000000002155000
>> 0x0000000000019488 0x0000000000019488 RW 200000
>> LOAD 0x000000000156f000 0xffffffff8216f000 0x000000000216f000
>> 0x0000000000122000 0x0000000000eb4000 RWE 200000
>> NOTE 0x0000000000ca0248 0xffffffff81aa0248 0x0000000001aa0248
>> 0x0000000000000024 0x0000000000000024 4
>>
>> Section to Segment mapping:
>> Segment Sections...
>> 00 .text .notes __ex_table .rodata __bug_table .pci_fixup
>> .builtin_fw .tracedata __ksymtab __ksymtab_gpl __ksymtab_strings
>> __param __modver
>> 01 .data .vvar
>> 02 .data..percpu
>> 03 .init.text .altinstr_aux .init.data .x86_cpu_dev.init
>> .altinstructions .altinstr_replacement .iommu_table .apicdrivers
>> .exit.text .smp_locks .bss .brk
>> 04 .notes
>>
>> The first load (containing .rodata) is "R E".
>
> Aah, right, I think thats because the program headers are specified
> explicitly in arch/x86/kernel/vmlinux.lds.S:
>
> PHDRS {
> text PT_LOAD FLAGS(5); /* R_E */
> data PT_LOAD FLAGS(6); /* RW_ */
> #ifdef CONFIG_X86_64
> #ifdef CONFIG_SMP
> percpu PT_LOAD FLAGS(6); /* RW_ */
> #endif
> init PT_LOAD FLAGS(7); /* RWE */
> #endif
> note PT_NOTE FLAGS(0); /* ___ */
> }
Ah-ha! That solves that mystery for me. :)
> The bit I was missing is that RO_DATA() is after .text, but before
> .data, so counts as part of the PT_LOAD program header for text.
Right, originally, it was so that there could be a single read-only
mapping covering both, but ultimately it doesn't matter now since they
can't share a mapping anyway: text needs to be read-only and
executable and rodata needs to be read-only and non-executable.
>> But, the point is: the kernel is what sets up the permissions, so the
>> flags are ignored anyway.
>
> Indeed.
>
> Thanks for your patience working through this stuff with me :)
No problem; I learned some stuff too. :)
-Kees
--
Kees Cook
Nexus Security
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