Re: [PATCH 0/9] mm: Hardened usercopy

[Ard Biesheuvel]

On 9 July 2016 at 04:22, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 07/06/2016 03:25 PM, Kees Cook wrote:
>>
>> Hi,
>>
>> This is a start of the mainline port of PAX_USERCOPY[1]. After I started
>> writing tests (now in lkdtm in -next) for Casey's earlier port[2], I
>> kept tweaking things further and further until I ended up with a whole
>> new patch series. To that end, I took Rik's feedback and made a number
>> of other changes and clean-ups as well.
>>
>> Based on my understanding, PAX_USERCOPY was designed to catch a few
>> classes of flaws around the use of copy_to_user()/copy_from_user(). These
>> changes don't touch get_user() and put_user(), since these operate on
>> constant sized lengths, and tend to be much less vulnerable. There
>> are effectively three distinct protections in the whole series,
>> each of which I've given a separate CONFIG, though this patch set is
>> only the first of the three intended protections. (Generally speaking,
>> PAX_USERCOPY covers what I'm calling CONFIG_HARDENED_USERCOPY (this) and
>> CONFIG_HARDENED_USERCOPY_WHITELIST (future), and PAX_USERCOPY_SLABS covers
>> CONFIG_HARDENED_USERCOPY_SPLIT_KMALLOC (future).)
>>
>> This series, which adds CONFIG_HARDENED_USERCOPY, checks that objects
>> being copied to/from userspace meet certain criteria:
>> - if address is a heap object, the size must not exceed the object's
>>   allocated size. (This will catch all kinds of heap overflow flaws.)
>> - if address range is in the current process stack, it must be within the
>>   current stack frame (if such checking is possible) or at least entirely
>>   within the current process's stack. (This could catch large lengths that
>>   would have extended beyond the current process stack, or overflows if
>>   their length extends back into the original stack.)
>> - if the address range is part of kernel data, rodata, or bss, allow it.
>> - if address range is page-allocated, that it doesn't span multiple
>>   allocations.
>> - if address is within the kernel text, reject it.
>> - everything else is accepted
>>
>> The patches in the series are:
>> - The core copy_to/from_user() checks, without the slab object checks:
>>         1- mm: Hardened usercopy
>> - Per-arch enablement of the protection:
>>         2- x86/uaccess: Enable hardened usercopy
>>         3- ARM: uaccess: Enable hardened usercopy
>>         4- arm64/uaccess: Enable hardened usercopy
>>         5- ia64/uaccess: Enable hardened usercopy
>>         6- powerpc/uaccess: Enable hardened usercopy
>>         7- sparc/uaccess: Enable hardened usercopy
>> - The heap allocator implementation of object size checking:
>>         8- mm: SLAB hardened usercopy support
>>         9- mm: SLUB hardened usercopy support
>>
>> Some notes:
>>
>> - This is expected to apply on top of -next which contains fixes for the
>>   position of _etext on both arm and arm64.
>>
>> - I couldn't detect a measurable performance change with these features
>>   enabled. Kernel build times were unchanged, hackbench was unchanged,
>>   etc. I think we could flip this to "on by default" at some point.
>>
>> - The SLOB support extracted from grsecurity seems entirely broken. I
>>   have no idea what's going on there, I spent my time testing SLAB and
>>   SLUB. Having someone else look at SLOB would be nice, but this series
>>   doesn't depend on it.
>>
>> Additional features that would be nice, but aren't blocking this series:
>>
>> - Needs more architecture support for stack frame checking (only x86 now).
>>
>>
>
> Even with the SLUB fixup I'm still seeing this blow up on my arm64 system.
> This is a
> Fedora rawhide kernel + the patches
>
> [ 0.666700] usercopy: kernel memory exposure attempt detected from
> fffffc0008b4dd58 (<kernel text>) (8 bytes)
> [ 0.666720] CPU: 2 PID: 79 Comm: modprobe Tainted: G        W
> 4.7.0-0.rc6.git1.1.hardenedusercopy.fc25.aarch64 #1
> [ 0.666733] Hardware name: AppliedMicro Mustang/Mustang, BIOS 1.1.0 Nov 24
> 2015
> [ 0.666744] Call trace:
> [ 0.666756] [<fffffc0008088a20>] dump_backtrace+0x0/0x1e8
> [ 0.666765] [<fffffc0008088c2c>] show_stack+0x24/0x30
> [ 0.666775] [<fffffc0008455344>] dump_stack+0xa4/0xe0
> [ 0.666785] [<fffffc000828d874>] __check_object_size+0x6c/0x230
> [ 0.666795] [<fffffc00083a5748>] create_elf_tables+0x74/0x420
> [ 0.666805] [<fffffc00082fb1f0>] load_elf_binary+0x828/0xb70
> [ 0.666814] [<fffffc0008298b4c>] search_binary_handler+0xb4/0x240
> [ 0.666823] [<fffffc0008299864>] do_execveat_common+0x63c/0x950
> [ 0.666832] [<fffffc0008299bb4>] do_execve+0x3c/0x50
> [ 0.666841] [<fffffc00080e3720>] call_usermodehelper_exec_async+0xe8/0x148
> [ 0.666850] [<fffffc0008084a80>] ret_from_fork+0x10/0x50
>
> This happens on every call to execve. This seems to be the first
> copy_to_user in
> create_elf_tables. I didn't get a chance to debug and I'm going out of town
> all of next week so all I have is the report unfortunately. config attached.
>

This is a known issue, and a fix is already queued for v4.8 in the arm64 tree:

9fdc14c55c arm64: mm: fix location of _etext [0]

which moves _etext up in the linker script so that it does not cover .rodata

ARM was suffering from the same problem, and Kees proposed a fix for
it. I don't know what the status of that patch is, though.

Note that on arm64, we have

  #define ELF_PLATFORM            ("aarch64")

which explains why k_platform points into .rodata in this case. On
ARM, it points to a writable string (as the code quoted by Rik shows),
so there it will likely explode elsewhere without the linker script
fix.

[0] https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/commit/?h=for-next/core&id=9fdc14c55c

-- 
Ard.