linux-kernel - Re: [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use

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Date:   Tue, 14 Jan 2020 13:32:50 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-snps-arc@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC 1/4] asm-generic/uaccess: don't define inline functions if
 noinline lib/* in use

On Tue, Jan 14, 2020 at 12:09 PM Vineet Gupta
<Vineet.Gupta1@...opsys.com> wrote:
>
> There are 2 generic varaints of strncpy_from_user() / strnlen_user()
>  (1). inline version in asm-generic/uaccess.h

I think we should get rid of this entirely. It's just a buggy garbage
implementation that nobody should ever actually use.

It does just about everything wrong that you *can* do, wrong,
including doing the NUL-filling termination of standard strncpy() that
"strncpy_from_user()" doesn't actually do.

So:

 - the asm-generic/uaccess.h __strncpy_from_user() function is just
horribly wrong

 - the generic/uaccess.h version of strncpy_from_user() shouldn't be
an inline function either, since the only thing it can do inline is
the bogus one-byte access check that _barely_ makes security work (you
also need to have a guard page to _actually_ make it work, and I'm not
atr all convinced that people do).

the whole thing is just broken and should be removed from a header file.

>  (2). optimized word-at-a-time version in lib/*

That is - outside of the original x86 strncpy_from_user() - the only
copy of this function that historically gets all the corner cases
right. And even those we've gotten wrong occasionally.

I would suggest that anybody who uses asm-generic/uaccess.h needs to
simply use the generic library version.

             Linus