Date: Fri, 12 Jan 2018 12:58:15 -0600 From: Josh Poimboeuf <jpoimboe@...hat.com> To: Dan Williams <dan.j.williams@...el.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 10:21:43AM -0800, Dan Williams wrote: > > 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. Converting your example code to assembly: call access_ok # no speculation which started before this point is allowed to continue past this point test %rax, %rax jne error_path dereference_uptr: (do nefarious things with the user pointer) error_path: mov -EINVAL, %rax ret So the CPU is still free to speculately execute the dereference_uptr block because the lfence was before the 'jne error_path' branch. -- Josh
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