Date: Wed, 17 Jan 2018 14:17:26 +0000 From: Alan Cox <alan@...ux.intel.com> To: Linus Torvalds <torvalds@...ux-foundation.org>, 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, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, the arch/x86 maintainers <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>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths On Tue, 2018-01-16 at 14:41 -0800, Linus Torvalds wrote: > > > On Jan 16, 2018 14:23, "Dan Williams" <dan.j.williams@...el.com> > wrote: > > That said, for get_user specifically, can we do something even > > cheaper. Dave H. reminds me that any valid user pointer that gets > > past > > the address limit check will have the high bit clear. So instead of > > calculating a mask, just unconditionally clear the high bit. It > > seems > > worse case userspace can speculatively leak something that's > > already > > in its address space. > > That's not at all true. > > The address may be a kernel address. That's the whole point of > 'set_fs()'. Can we kill off the remaining users of set_fs() ? Alan
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