Date: Fri, 12 Jan 2018 10:21:43 -0800 From: Dan Williams <dan.j.williams@...el.com> To: Josh Poimboeuf <jpoimboe@...hat.com> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>, Kees Cook <keescook@...omium.org>, kernel-hardening@...ts.openwall.com, X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>, Al Viro <viro@...iv.linux.org.uk>, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Alan Cox <alan@...ux.intel.com> Subject: Re: [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM_IFENCE On Fri, Jan 12, 2018 at 9:51 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote: > On Thu, Jan 11, 2018 at 04:47:02PM -0800, Dan Williams wrote: >> For 'get_user' paths, do not allow the kernel to speculate on the value >> of a user controlled pointer. In addition to the 'stac' instruction for >> Supervisor Mode Access Protection, an 'ifence' causes the 'access_ok' >> result to resolve in the pipeline before the cpu might take any >> speculative action on the pointer value. > > So I understand the need to "patch first and ask questions later". I > also understand that usercopy is an obvious attack point for speculative > bugs. However, I'm still hopelessly confused about what exactly this > patch (and the next one) are supposed to accomplish. > > I can't figure out if: > > a) I'm missing something completely obvious; > b) this is poorly described; or > c) it doesn't actually fix/protect/harden anything. > > The commit log doesn't help me at all. In fact, it confuses me more. > For example, this paragraph: > >> Since this is a major kernel interface that deals with user controlled >> data, the '__uaccess_begin_nospec' mechanism will prevent speculative >> execution past an 'access_ok' permission check. While speculative >> execution past 'access_ok' is not enough to lead to a kernel memory >> leak, it is a necessary precondition. > > That just sounds wrong. What if the speculation starts *after* the > access_ok() check? Then the barrier has no purpose. > > Most access_ok/get_user/copy_from_user calls are like this: > > if (copy_from_user(...uptr..)) /* or access_ok() or get_user() */ > return -EFAULT; > > So in other words, the usercopy function is called *before* the branch. > > But to halt speculation, the lfence needs to come *after* the branch. > So putting lfences *before* the branch doesn't solve anything. > > So what am I missing? We're trying to prevent a pointer under user control from being de-referenced inside the kernel, before we know it has been limited to something safe. In the following sequence the branch we are worried about speculating is the privilege check: if (access_ok(uptr)) /* <--- Privelege Check */ if (copy_from_user_(uptr)) The cpu can speculatively skip that access_ok() check and cause a read of kernel memory.
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