Date: Sat, 9 Jul 2016 10:25:00 +0200 From: Ard Biesheuvel <ard.biesheuvel@...aro.org> To: Laura Abbott <labbott@...hat.com> Cc: Kees Cook <keescook@...omium.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Rik van Riel <riel@...hat.com>, Casey Schaufler <casey@...aufler-ca.com>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Michael Ellerman <mpe@...erman.id.au>, Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, "David S. Miller" <davem@...emloft.net>, "x86@...nel.org" <x86@...nel.org>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Mathias Krause <minipli@...glemail.com>, Jan Kara <jack@...e.cz>, Vitaly Wool <vitalywool@...il.com>, Andrea Arcangeli <aarcange@...hat.com>, Dmitry Vyukov <dvyukov@...gle.com>, Laura Abbott <labbott@...oraproject.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>, linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, sparclinux@...r.kernel.org, "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH 0/9] mm: Hardened usercopy On 9 July 2016 at 04:22, Laura Abbott <labbott@...hat.com> wrote: > On 07/06/2016 03:25 PM, Kees Cook wrote: >> >> Hi, >> >> This is a start of the mainline port of PAX_USERCOPY. After I started >> writing tests (now in lkdtm in -next) for Casey's earlier port, 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  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.  https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/commit/?h=for-next/core&id=9fdc14c55c -- Ard.
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