linux-kernel - Re: [uaccess] 780464aed0: WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen

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Message-Id: <20190304180610.2d4f6f08d9ad89d6abae3597@kernel.org>
Date:   Mon, 4 Mar 2019 18:06:10 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <lkp@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Shuah Khan <shuah@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>, yhs@...com, lkp@...org
Subject: Re: [uaccess] 780464aed0:
 WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x

On Sun, 3 Mar 2019 18:37:59 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, Mar 3, 2019 at 5:14 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > I think it comes from WARN_ON_ONCE(!segment_eq(get_fs(), USER_DS)) in
> > user_access_ok(). The call trace shows that strndup_user might be called
> > from kernel daemon context.
> 
> Ahh, yes.
> 
> We've had this before. We've gotten rid of the actual "use system
> calls", but we still have some of the init sequence in particular just
> calling the wrappers instead.

Are those safe if we are in init sequence?

> 
> And yes, ksys_mount() takes __user pointers.
> 
> It would be a lot better to use "do_mount()", which is the interface
> that takes actual "char *" pointers.

Unfortunately, it still takes a __user pointer.

long do_mount(const char *dev_name, const char __user *dir_name,
                const char *type_page, unsigned long flags, void *data_page)

So what we need is

long do_mount(const char *dev_name, struct path *dir_path,
                const char *type_page, unsigned long flags, void *data_page)

or introduce kern_do_mount()?

Since devtmpfsd calls ksys_chdir() and ksys_chroot(), we need to replace
those too. Fortunately, it seems that the last part which we have to fix.

Thank you,

> 
>                     Linus


-- 
Masami Hiramatsu <mhiramat@...nel.org>