[PATCH 0/9] mm: Hardened usercopy

Sat Jul 9 22:58:22 AEST 2016

On Sat, Jul 9, 2016 at 1:25 AM, Ard Biesheuvel <ard.biesheuvel at linaro.org>
wrote:

> On 9 July 2016 at 04:22, Laura Abbott <labbott at redhat.com> 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.
>

Ugh, I completely missed that note about the patch on arm64. Sorry for the
noise.

Thanks,
Laura
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