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

[Laura Abbott <labbott@redhat.com>]

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.

Thanks,
Laura