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

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Date:   Tue, 5 Mar 2019 10:07:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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 Tue, Mar 05, 2019 at 11:36:35AM +0900, Masami Hiramatsu wrote:
> I think the better way to do this is allowing strncpy_from_user()
O
> if some conditions are match, like
> 
>  - strncpy_from_user() will be able to copy user memory with set_fs(USER_DS)
>  - strncpy_from_user() can copy kernel memory with set_fs(KERNEL_DS)
>  - strncpy_from_user() can access unsafe memory in IRQ context if
>    pagefault is disabled.
> 
> This is almost done, except for CONFIG_DEBUG_ATOMIC_SLEEP=y on x86.
> 
> So, what about adding a condition to WARN_ON_IN_IRQ() like below
> instead of introducing user_access_ok() ?
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 780f2b42c8ef..ec0f0b74c9ab 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -70,7 +70,7 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
>  })
>  
>  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> -# define WARN_ON_IN_IRQ()      WARN_ON_ONCE(!in_task())
> +# define WARN_ON_IN_IRQ()      WARN_ON_ONCE(pagefault_disabled() && !in_task())

That doesn't make any kind of sense to me; see faulthandler_disabled().
IOW. interrupt (and any atomic context really) won't take faults anyway.

I dislike that whole KERNEL_DS thing, but obviously that's not something
that's going away.

Would something like:

	WARN_ON_ONCE(!(in_task || segment_eq(get_fs(), USER_DS)))

Work? Then we allow KERNEL_DS in task context, but for interrupt and
others require USER_DS.