Re: [PATCH v2 1/2] bpf: extend stackmap to save binary_build_id+offset instead of address

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 06, 2018 at 10:09:13AM -0800, Song Liu wrote:
> +/* Parse build ID of ELF file mapped to vma */
> +static int stack_map_get_build_id(struct vm_area_struct *vma,
> +				  unsigned char *build_id)
> +{
> +	Elf32_Ehdr *ehdr;
> +	struct page *page;
> +	int ret;
> +
> +	/*
> +	 * vm_start is user memory, so we need to be careful with it.
> +	 * We don't want too many copy_from_user to reduce overhead.
> +	 * Most ELF file is at least one page long, and the build_id
> +	 * is stored in the first page. Therefore, we limit the search of
> +	 * build_id to the first page only. This can be made safe with
> +	 * get_user_pages_fast(). If the file is smaller than PAGE_SIZE,
> +	 * or the build_id is not in the first page, the look up fails.
> +	 */
> +	if (vma->vm_end - vma->vm_start < PAGE_SIZE ||
> +	    vma->vm_start & (PAGE_SIZE - 1))	/* page aligned */
> +		return -EINVAL;
> +
> +	if (get_user_pages_fast(vma->vm_start, 1, 0, &page) != 1)
> +		return -EFAULT;
> +
> +	ret = -EINVAL;
> +	ehdr = (Elf32_Ehdr *)vma->vm_start;
> +

You're still directly accessing a user pointer afaict. This will
insta-fault on SMAP enabled hardware due to the lack of STAC (or PAN on
ARM).

You _really_ cannot just access user pointers without the proper APIs.

Did you maybe mean to use:

	ehdr = (Elf32_Ehdr *)page_address(page);

?

> +	/* compare magic x7f "ELF" */
> +	if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
> +		goto out;
> +
> +	/* only support executable file and shared object file */
> +	if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
> +		goto out;
> +
> +	if (ehdr->e_ident[EI_CLASS] == 1)
> +		ret = stack_map_get_build_id_32(vma, build_id);
> +	else if (ehdr->e_ident[EI_CLASS] == 2)
> +		ret = stack_map_get_build_id_64(vma, build_id);
> +out:
> +	put_page(page);
> +	return ret;
> +}