Date: Wed, 4 Oct 2017 10:28:13 -0700 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Kees Cook <keescook@...omium.org> Cc: "Tobin C. Harding" <me@...in.cc>, Greg KH <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, William Roberts <william.c.roberts@...el.com>, Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com> Subject: Re: [RFC V2 4/6] lib: vsprintf: default kptr_restrict to the maximum value On Wed, Oct 4, 2017 at 10:08 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > > So I honestly doubt the value of kptr_restrict. Any *sane* policy > pretty much has to be in the caller, and by thinking about what you > print out. IOW, things like proc_pid_wchan(). Looking at /proc/kallsyms is actually a prime example of this. IOW, the old "open /proc/kallsyms as a normal user, then make it stdin for some suid-root program that can be fooled to output it probably works on it. So kptr_restrict ends up being entirely the wrong thing to do there. The only value in kptr_restrict ends up being as a complete hack, where you say "I trust nobody" and make %p almost entirely useless. And as mentioned, that will just make people use %x instead, or randomly sprinkle the new "I didn't really mean this" modifiers like the already discussed pr_debug() case. So even when kptr_restrict "works", it ends up just fighting itself. And most of the time it just doesn't work. Linus
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