linux-kernel - Re: [PATCH v7 2/6] uaccess: Add non-pagefault user-space read functions

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Date:   Thu, 9 May 2019 11:14:08 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        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
Subject: Re: [PATCH v7 2/6] uaccess: Add non-pagefault user-space read
 functions

* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> +static __always_inline long
> +probe_read_common(void *dst, const void __user *src, size_t size)
> +{
> +	long ret;
> +
> +	pagefault_disable();
> +	ret = __copy_from_user_inatomic(dst, src, size);
> +	pagefault_enable();
> +
> +	return ret ? -EFAULT : 0;
> +}

Empty line before return statement: good.

> +long __weak probe_user_read(void *dst, const void __user *src, size_t size)
> +    __attribute__((alias("__probe_user_read")));
> +
> +long __probe_user_read(void *dst, const void __user *src, size_t size)
> +{
> +	long ret = -EFAULT;
> +	mm_segment_t old_fs = get_fs();
> +
> +	set_fs(USER_DS);
> +	if (access_ok(src, size))
> +		ret = probe_read_common(dst, src, size);
> +	set_fs(old_fs);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(probe_user_read);

No empty line before return statement: not good.

> +long strncpy_from_unsafe_user(char *dst, const void __user *unsafe_addr,
> +			      long count)
> +{
> +	mm_segment_t old_fs = get_fs();
> +	long ret;
> +
> +	if (unlikely(count <= 0))
> +		return 0;
> +
> +	set_fs(USER_DS);
> +	pagefault_disable();
> +	ret = strncpy_from_user(dst, unsafe_addr, count);
> +	pagefault_enable();
> +	set_fs(old_fs);
> +	if (ret >= count) {
> +		ret = count;
> +		dst[ret - 1] = '\0';
> +	} else if (ret > 0)
> +		ret++;
> +	return ret;
> +}

Ditto. Also unbalanced curly braces.

> +
> +/**
> + * strnlen_unsafe_user: - Get the size of a user string INCLUDING final NUL.
> + * @unsafe_addr: The string to measure.
> + * @count: Maximum count (including NUL character)
> + *
> + * Get the size of a NUL-terminated string in user space without pagefault.
> + *
> + * Returns the size of the string INCLUDING the terminating NUL.

These phrases exist:

 'Terminating NULL'
 'NULL character'

And we also sometimes talk about 'nil' - but I don't think there's such 
thing as a 'NUL character'?

I realize that this was probably cloned from existing lib/strnlen_user.c 
code, but still. ;-)

> + *
> + * If the string is too long, returns a number larger than @count. User
> + * has to check the return value against "> count".
> + * On exception (or invalid count), returns 0.
> + *
> + * Unlike strnlen_user, this can be used from IRQ handler etc. because
> + * it disables pagefaults.

'can be used from IRQ handlers'

> + */
> +long strnlen_unsafe_user(const void __user *unsafe_addr, long count)
> +{
> +	mm_segment_t old_fs = get_fs();
> +	int ret;
> +
> +	set_fs(USER_DS);
> +	pagefault_disable();
> +	ret = strnlen_user(unsafe_addr, count);
> +	pagefault_enable();
> +	set_fs(old_fs);
> +	return ret;
> +}

Same problem as before.

Thanks,

	Ingo