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;
> +}
