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Date: Fri, 8 Jul 2016 19:22:50 -0700
From: Laura Abbott <>
To: Kees Cook <>,
Cc: Rik van Riel <>, Casey Schaufler <>,
 PaX Team <>, Brad Spengler <>,
 Russell King <>,
 Catalin Marinas <>, Will Deacon
 <>, Ard Biesheuvel <>,
 Benjamin Herrenschmidt <>,
 Michael Ellerman <>, Tony Luck <>,
 Fenghua Yu <>, "David S. Miller" <>,, Christoph Lameter <>,
 Pekka Enberg <>, David Rientjes <>,
 Joonsoo Kim <>,
 Andrew Morton <>, Andy Lutomirski
 <>, Borislav Petkov <>,
 Mathias Krause <>, Jan Kara <>,
 Vitaly Wool <>, Andrea Arcangeli <>,
 Dmitry Vyukov <>, Laura Abbott
Subject: Re: [PATCH 0/9] mm: Hardened usercopy

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
> 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.


View attachment "hardened_copy_config" of type "text/plain" (162541 bytes)

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