Re: [PATCH v4 00/12] mm: Hardened usercopy

[Kees Cook]

On Fri, Jul 22, 2016 at 5:36 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 07/20/2016 01:26 PM, Kees Cook wrote:
>>
>> Hi,
>>
>> [This is now in my kspp -next tree, though I'd really love to add some
>> additional explicit Tested-bys, Reviewed-bys, or Acked-bys. If you've
>> looked through any part of this or have done any testing, please consider
>> sending an email with your "*-by:" line. :)]
>>
>> This is a start of the mainline port of PAX_USERCOPY[1]. After 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, Laura, and other people's feedback along with
>> additional changes and clean-ups.
>>
>> Based on my understanding, PAX_USERCOPY was designed to catch a
>> few classes of flaws (mainly bad bounds checking) 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
>>   a valid 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 (excepting Reserved and CMA pages).
>> - if address is within the kernel text, reject it.
>> - everything else is accepted
>>
>> The patches in the series are:
>> - Support for examination of CMA page types:
>>         1- mm: Add is_migrate_cma_page
>> - Support for arch-specific stack frame checking (which will likely be
>>   replaced in the future by Josh's more comprehensive unwinder):
>>         2- mm: Implement stack frame object validation
>> - The core copy_to/from_user() checks, without the slab object checks:
>>         3- mm: Hardened usercopy
>> - Per-arch enablement of the protection:
>>         4- x86/uaccess: Enable hardened usercopy
>>         5- ARM: uaccess: Enable hardened usercopy
>>         6- arm64/uaccess: Enable hardened usercopy
>>         7- ia64/uaccess: Enable hardened usercopy
>>         8- powerpc/uaccess: Enable hardened usercopy
>>         9- sparc/uaccess: Enable hardened usercopy
>>        10- s390/uaccess: Enable hardened usercopy
>> - The heap allocator implementation of object size checking:
>>        11- mm: SLAB hardened usercopy support
>>        12- 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, though it has some conflicts
>>   with KASAN that should be trivial to fix up. Also in -next are the
>>   tests for this protection (in lkdtm), prefixed with USERCOPY_.
>>
>> - 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, but
>>   for now, I'm leaving it off until I can get some more definitive
>>   measurements. I would love if someone with greater familiarity with
>>   perf could give this a spin and report results.
>>
>> - 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,
>>   but it seems Josh will have a good solution for this soon).
>>
>>
>> Thanks!
>>
>> -Kees
>>
>> [1] https://grsecurity.net/download.php "grsecurity - test kernel patch"
>> [2] http://www.openwall.com/lists/kernel-hardening/2016/05/19/5
>>
>> v4:
>> - handle CMA pages, labbott
>> - update stack checker comments, labbott
>> - check for vmalloc addresses, labbott
>> - deal with KASAN in -next changing arm64 copy*user calls
>> - check for linear mappings at runtime instead of via CONFIG
>>
>> v3:
>> - switch to using BUG for better Oops integration
>> - when checking page allocations, check each for Reserved
>> - use enums for the stack check return for readability
>>
>> v2:
>> - added s390 support
>> - handle slub red zone
>> - disallow writes to rodata area
>> - stack frame walker now CONFIG-controlled arch-specific helper
>>
>
> Do you have/plan to have LKDTM or the like tests for this? I started
> reviewing
> the slub code and was about to write some test cases for myself. I did that
> for CMA as well which is a decent indicator these should all go somewhere.

Yeah, there is an entire section of tests in lkdtm for the usercopy
protection. I didn't add anything for CMA or multipage allocations
yet, though. Feel free to add those if you have a moment! :) It's on
my todo list.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security