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Date: Wed, 1 Nov 2017 14:13:45 -0700 From: Laura Abbott <labbott@...hat.com> To: Mark Rutland <mark.rutland@....com> Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com, Catalin Marinas <catalin.marinas@....com>, Kees Cook <keescook@...omium.org>, Will Deacon <will.deacon@....com> Subject: Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks On 11/01/2017 05:05 AM, Mark Rutland wrote: > On Tue, Oct 31, 2017 at 04:56:39PM -0700, Laura Abbott wrote: >> On 10/26/2017 02:09 AM, Mark Rutland wrote: >>> In Prague, Kees mentioned that it would be nice to have a mechanism to >>> catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2] >>> issue with unsafe_put_user() in waitid(). >>> >>> These patches allow an optional access_ok() check to be dropped in >>> arm64's __{get,put}_user() primitives. These will then BUG() if a bad >>> user pointer is passed (which should only happen in the absence of an >>> earlier access_ok() check). > >> Turning on the option fails as soon as we hit userspace. On my buildroot >> based environment I get the help text for ld.so (????) and then a message >> about attempting to kill init. > > Ouch. Thanks for the report, and sorry about this. > > The problem is that I evaluate the ptr argument twice in > __{get,put}_user(), and this may have side effects. > > e.g. when the ELF loader does things like: > > __put_user((elf_addr_t)p, sp++) > > ... we increment sp twice, and write to the wrong user address, leaving > sp corrupt. > > I have an additional patch [1] to fix this, which is in my > arm64/access-ok branch [2]. > > Thanks, > Mark. > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/access-ok&id=ebb7ff83eb53b8810395d5cf48712a4ae6d678543 > [2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/access-ok > Thanks, the updated patch works. I wrote an LKDTM test to verify the expected behavior (__{get,put}_user panic whereas {get,put}_user do not). You're welcome to add Tested-by or I can wait for v2. Thanks, Laura
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