linux-kernel - Re: [PATCH v9 2/3] mm: add a field to store names for private anonymous memory

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Date:   Fri, 3 Sep 2021 14:47:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, ccross@...gle.com,
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Subject: Re: [PATCH v9 2/3] mm: add a field to store names for private
 anonymous memory

(Sorry, a few more things jumped out at me when I looked again...)

On Thu, Sep 02, 2021 at 04:18:12PM -0700, Suren Baghdasaryan wrote:
> [...]
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 72c7639e3c98..25118902a376 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2299,6 +2299,64 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
>  
>  #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
>  
> +#ifdef CONFIG_MMU
> +
> +#define ANON_VMA_NAME_MAX_LEN	256
> +
> +static inline bool is_valid_name_char(char ch)
> +{
> +	/* printable ascii characters, except [ \ ] */
> +	return (ch > 0x1f && ch < 0x5b) || (ch > 0x5d && ch < 0x7f);
> +}

In the back of my mind, I feel like disallowing backtick would be nice,
but then if $, (, and ) are allowed, it doesn't matter, and that seems
too limiting. :)

> +
> +static int prctl_set_vma(unsigned long opt, unsigned long addr,
> +			 unsigned long size, unsigned long arg)
> +{
> +	struct mm_struct *mm = current->mm;
> +	const char __user *uname;
> +	char *name, *pch;
> +	int error;
> +
> +	switch (opt) {
> +	case PR_SET_VMA_ANON_NAME:
> +		uname = (const char __user *)arg;
> +		if (!uname) {
> +			/* Reset the name */
> +			name = NULL;
> +			goto set_name;
> +		}
> +
> +		name = strndup_user(uname, ANON_VMA_NAME_MAX_LEN);
> +
> +		if (IS_ERR(name))
> +			return PTR_ERR(name);
> +
> +		for (pch = name; *pch != '\0'; pch++) {
> +			if (!is_valid_name_char(*pch)) {
> +				kfree(name);
> +				return -EINVAL;
> +			}
> +		}
> +set_name:
> +		mmap_write_lock(mm);
> +		error = madvise_set_anon_name(mm, addr, size, name);
> +		mmap_write_unlock(mm);
> +		kfree(name);
> +		break;

This is a weird construct with a needless goto. Why not:

	switch (opt) {
	case PR_SET_VMA_ANON_NAME:
		uname = (const char __user *)arg;
		if (uname) {
			name = strndup_user(uname, ANON_VMA_NAME_MAX_LEN);
			if (IS_ERR(name))
				return PTR_ERR(name);

			for (pch = name; *pch != '\0'; pch++) {
				if (!is_valid_name_char(*pch)) {
					kfree(name);
					return -EINVAL;
				}
			}
		} else {
			/* Reset the name */
			name = NULL;
		}
		mmap_write_lock(mm);
		error = madvise_set_anon_name(mm, addr, size, name);
		mmap_write_unlock(mm);
		kfree(name);
		break;


-- 
Kees Cook