Re: Fw: signed kernel modules?

[David Howells <dhowells@redhat.com>]

> Let me make this clear: I refuse to include any solution which doesn't
> protect against accidental, as well as deliberate, corruption.  This
> means your "canonicalization" code has to be very, very paranoid about
> not trusting the data until the signature is verified.  The current code
> does very simple checks then completely trusts the module contents,
> especially the section headers: to make signatures worth anything, your
> code must not do this.

I agree that checks should be made on ELF before signature checking is
performed, but I was trying to develop it one step at a time. However, since
you asked so nicely, I've added the attached to my implementation.

David

---

struct module_verify_data {
	struct crypto_tfm	*sha1_tfm;
	const void		*buffer;
	const Elf_Ehdr		*hdr;
	const Elf_Shdr		*sections;
	const Elf_Sym		*symbols;
	const char		*secstrings;
	const char		*strings;
	size_t			*secsizes;
	size_t			size;
	size_t			nsects;
	size_t			nsyms;
	size_t			nstrings;
	size_t			signed_size;
	int			*canonlist;
	int			*canonmap;
	int			sig_index;
	uint8_t			csum;
};

/*****************************************************************************/
/*
 * verify the ELF structure of a module
 */
static int module_verify_elf(struct module_verify_data *mvdata)
{
	const Elf_Ehdr *hdr = mvdata->hdr;
	const Elf_Shdr *section, *section2, *secstop;
	const Elf_Rela *relas, *rela, *relastop;
	const Elf_Rel *rels, *rel, *relstop;
	const Elf_Sym *symbol, *symstop;
	size_t size, sssize, *secsize, tmp, tmp2;
	int line;

	size = mvdata->size;
	mvdata->nsects = hdr->e_shnum;

#define elfcheck(X) \
do { if (unlikely(!(X))) { line = __LINE__; goto elfcheck_error; } } while(0)

#define seccheck(X) \
do { if (unlikely(!(X))) { line = __LINE__; goto seccheck_error; } } while(0)

#define symcheck(X) \
do { if (unlikely(!(X))) { line = __LINE__; goto symcheck_error; } } while(0)

#define relcheck(X) \
do { if (unlikely(!(X))) { line = __LINE__; goto relcheck_error; } } while(0)

#define relacheck(X) \
do { if (unlikely(!(X))) { line = __LINE__; goto relacheck_error; } } while(0)

	/* validate the ELF header */
	elfcheck(hdr->e_ehsize < size);
	elfcheck(hdr->e_entry == 0);
	elfcheck(hdr->e_phoff == 0);
	elfcheck(hdr->e_phnum == 0);

	elfcheck(hdr->e_shoff < size);
	elfcheck(hdr->e_shoff >= hdr->e_ehsize);
	elfcheck((hdr->e_shoff & (sizeof(long) - 1)) == 0);
	elfcheck(hdr->e_shstrndx > 0);
	elfcheck(hdr->e_shstrndx < hdr->e_shnum);

	tmp = (size_t) hdr->e_shentsize * (size_t) hdr->e_shnum;
	elfcheck(tmp < size - hdr->e_shoff);

	/* allocate a table to hold in-file section sizes */
	mvdata->secsizes = kmalloc(hdr->e_shnum * sizeof(size_t), GFP_KERNEL);
	if (!mvdata->secsizes)
		return -ENOMEM;

	memset(mvdata->secsizes, 0, hdr->e_shnum * sizeof(size_t));

	/* validate the ELF section headers */
	mvdata->sections = mvdata->buffer + hdr->e_shoff;
	secstop = mvdata->sections + mvdata->nsects;

	sssize = mvdata->sections[hdr->e_shstrndx].sh_size;
	elfcheck(sssize > 0);

	section = mvdata->sections;
	seccheck(section->sh_type == SHT_NULL);
	seccheck(section->sh_size == 0);
	seccheck(section->sh_offset == 0);

	secsize = mvdata->secsizes + 1;
	for (section++; section < secstop; secsize++, section++) {
		seccheck(section->sh_name < sssize);
		seccheck(section->sh_addr == 0);
		seccheck(section->sh_link < hdr->e_shnum);

		if (section->sh_entsize > 0)
			seccheck(section->sh_size % section->sh_entsize == 0);

		seccheck(section->sh_offset >= hdr->e_ehsize);
		seccheck(section->sh_offset < size);

		/* determine the section's in-file size */
		tmp = size - section->sh_offset;
		if (section->sh_offset < hdr->e_shoff)
			tmp = hdr->e_shoff - section->sh_offset;

		for (section2 = mvdata->sections + 1; section2 < secstop; section2++) {
			if (section->sh_offset < section2->sh_offset) {
				tmp2 = section2->sh_offset - section->sh_offset;
				if (tmp2 < tmp)
					tmp = tmp2;
			}
		}
		*secsize = tmp;

		_debug("Section %ld: %zx bytes at %lx\n",
		       section - mvdata->sections,
		       *secsize,
		       section->sh_offset);

		/* perform section type specific checks */
		switch (section->sh_type) {
		case SHT_NOBITS:
			break;

		case SHT_REL:
		case SHT_RELA:
			seccheck(section->sh_info > 0);
			seccheck(section->sh_info < hdr->e_shnum);
			/* fall through */

		default:
			/* most types of section must be contained entirely
			 * within the file */
			seccheck(section->sh_size <= *secsize);
			break;
		}
	}

	/* validate the ELF section names */
	section = &mvdata->sections[hdr->e_shstrndx];

	seccheck(section->sh_offset != hdr->e_shoff);

	mvdata->secstrings = mvdata->buffer + section->sh_offset;

	for (section = mvdata->sections + 1; section < secstop; section++) {
		const char *secname;
		tmp = sssize - section->sh_name;
		secname = mvdata->secstrings + section->sh_name;
		seccheck(secname[0] != 0);
		seccheck(memchr(secname, 0, tmp) != NULL);
	}

	/* look for various sections in the module */
	for (section = mvdata->sections + 1; section < secstop; section++) {
		switch (section->sh_type) {
		case SHT_SYMTAB:
			if (strcmp(mvdata->secstrings + section->sh_name,
				   ".symtab") == 0
			    ) {
				seccheck(mvdata->symbols == NULL);
				mvdata->symbols =
					mvdata->buffer + section->sh_offset;
				mvdata->nsyms =
					section->sh_size / sizeof(Elf_Sym);
				seccheck(section->sh_size > 0);
			}
			break;

		case SHT_STRTAB:
			if (strcmp(mvdata->secstrings + section->sh_name,
				   ".strtab") == 0
			    ) {
				seccheck(mvdata->strings == NULL);
				mvdata->strings =
					mvdata->buffer + section->sh_offset;
				sssize = mvdata->nstrings = section->sh_size;
				seccheck(section->sh_size > 0);
			}
			break;
		}
	}

	if (!mvdata->symbols) {
		printk("Couldn't locate module symbol table\n");
		goto format_error;
	}

	if (!mvdata->strings) {
		printk("Couldn't locate module strings table\n");
		goto format_error;
	}

	/* validate the symbol table */
	symstop = mvdata->symbols + mvdata->nsyms;

	symbol = mvdata->symbols;
	symcheck(ELF_ST_TYPE(symbol[0].st_info) == STT_NOTYPE);
	symcheck(symbol[0].st_shndx == SHN_UNDEF);
	symcheck(symbol[0].st_value == 0);
	symcheck(symbol[0].st_size == 0);

	for (symbol++; symbol < symstop; symbol++) {
		symcheck(symbol->st_name < sssize);
		symcheck(symbol->st_shndx < mvdata->nsects ||
			 symbol->st_shndx >= SHN_LORESERVE);
	}

	/* validate each relocation table as best we can */
	for (section = mvdata->sections + 1; section < secstop; section++) {
		section2 = mvdata->sections + section->sh_info;

		switch (section->sh_type) {
		case SHT_REL:
			rels = mvdata->buffer + section->sh_offset;
			relstop = mvdata->buffer + section->sh_offset + section->sh_size;

			for (rel = rels; rel < relstop; rel++) {
				relcheck(rel->r_offset < section2->sh_size);
				relcheck(ELF_R_SYM(rel->r_info) < mvdata->nsyms);
			}

			break;

		case SHT_RELA:
			relas = mvdata->buffer + section->sh_offset;
			relastop = mvdata->buffer + section->sh_offset + section->sh_size;

			for (rela = relas; rela < relastop; rela++) {
				relacheck(rela->r_offset < section2->sh_size);
				relacheck(ELF_R_SYM(rela->r_info) < mvdata->nsyms);
			}

			break;

		default:
			break;
		}
	}


	_debug("ELF okay\n");
	return 0;

 elfcheck_error:
	printk("Verify ELF error (assertion %d)\n", line);
	goto format_error;

 seccheck_error:
	printk("Verify ELF error [sec %ld] (assertion %d)\n",
	       section - mvdata->sections, line);
	goto format_error;

 symcheck_error:
	printk("Verify ELF error [sym %ld] (assertion %d)\n",
	       symbol - mvdata->symbols, line);
	goto format_error;

 relcheck_error:
	printk("Verify ELF error [sec %ld rel %ld] (assertion %d)\n",
	       section - mvdata->sections, rel - rels, line);
	goto format_error;

 relacheck_error:
	printk("Verify ELF error [sec %ld rela %ld] (assertion %d)\n",
	       section - mvdata->sections, rela - relas, line);
	goto format_error;

 format_error:
	return -ELIBBAD;

} /* end module_verify_elf() */