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SubjectRe: [PATCH] module: harden ELF info handling
Hi Frank,

Sorry for the delay. I've just gotten back from vacation :-)

+++ Frank van der Linden [21/12/20 23:49 +0000]:
>5fdc7db644 ("module: setup load info before module_sig_check()")
>moved the ELF setup, so that it was done before the signature
>check. This made the module name available to signature error
>messages.
>
>However, the checks for ELF correctness in setup_load_info
>are not sufficient to prevent bad memory references due to
>corrupted offset fields, indices, etc.
>
>So, there's a regression in behavior here: a corrupt and unsigned
>(or badly signed) module, which might previously have been rejected
>immediately, can now cause an oops/crash.
>
>Harden ELF handling for module loading by doing the following:
>
>- Move the signature check back up so that it comes before ELF
> initialization. It's best to do the signature check to see
> if we can trust the module, before using the ELF structures
> inside it. This also makes checks against info->len
> more accurate again, as this field will be reduced by the
> length of the signature in mod_check_sig().
>
> The module name is now once again not available for error
> messages during the signature check, but that seems like
> a fair tradeoff.

I vaguely remember that I had made the module name available in
response to a one-off request, IIRC someone had wanted the module name
logged to be able to figure out which module(s) had failed signature
verification. But I do agree with your line of reasoning, that we
should probably not access internal module structures until we have
verified that we can trust the module. It is a chicken and egg problem
unfortunately. Although, it is probably worth it to trade ease of
debugging for a more hardened approach.

>- Check if sections have offset / size fields that at least don't
> exceed the length of the module.
>
>- Check if sections have section name offsets that don't fall
> outside the section name table.
>
>- Add a few other sanity checks against invalid section indices,
> etc.
>
>This is not an exhaustive consistency check, but the idea is to
>at least get through the signature and blacklist checks without
>crashing because of corrupted ELF info, and to error out gracefully
>for most issues that would have caused problems later on.
>
>Fixes: 5fdc7db644 ("module: setup load info before module_sig_check()")
>Signed-off-by: Frank van der Linden <fllinden@amazon.com>
>---
> kernel/module.c | 143 +++++++++++++++++++++++++++++++++-----
> kernel/module_signature.c | 2 +-
> kernel/module_signing.c | 2 +-
> 3 files changed, 126 insertions(+), 21 deletions(-)
>
>diff --git a/kernel/module.c b/kernel/module.c
>index 4bf30e4b3eaa..ef7681a22a1a 100644
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -2964,7 +2964,7 @@ static int module_sig_check(struct load_info *info, int flags)
> }
>
> if (is_module_sig_enforced()) {
>- pr_notice("%s: loading of %s is rejected\n", info->name, reason);
>+ pr_notice("loading of %s is rejected\n", reason);

Small nit: Let's start with a capital letter perhaps? Just to be
consistent with the other log messages that don't start with a prefix.
Same goes for the other pr_err()s below.

> return -EKEYREJECTED;
> }
>
>@@ -2977,9 +2977,33 @@ static int module_sig_check(struct load_info *info, int flags)
> }
> #endif /* !CONFIG_MODULE_SIG */
>
>-/* Sanity checks against invalid binaries, wrong arch, weird elf version. */
>-static int elf_header_check(struct load_info *info)
>+static int validate_section_offset(struct load_info *info, Elf_Shdr *shdr)
> {
>+ unsigned long secend;
>+
>+ /*
>+ * Check for both overflow and offset/size being
>+ * too large.
>+ */
>+ secend = shdr->sh_offset + shdr->sh_size;
>+ if (secend < shdr->sh_offset || secend >= info->len)

Should this not be secend > info->len?

>+ return -ENOEXEC;
>+
>+ return 0;
>+}
>+
>+/*
>+ * Sanity checks against invalid binaries, wrong arch, weird elf version.
>+ *
>+ * Also do basic validity checks against section offsets and sizes, the
>+ * section name string table, and the indices used for it (sh_name).
>+ */
>+static int elf_validity_check(struct load_info *info)
>+{
>+ unsigned int i;
>+ Elf_Shdr *shdr, *strhdr;
>+ int err;
>+
> if (info->len < sizeof(*(info->hdr)))
> return -ENOEXEC;
>
>@@ -2989,11 +3013,78 @@ static int elf_header_check(struct load_info *info)
> || info->hdr->e_shentsize != sizeof(Elf_Shdr))
> return -ENOEXEC;
>
>+ /*
>+ * e_shnum is 16 bits, and sizeof(Elf_Shdr) is
>+ * known and small. So e_shnum * sizeof(Elf_Shdr)
>+ * will not overflow unsigned long on any platform.
>+ */
> if (info->hdr->e_shoff >= info->len
> || (info->hdr->e_shnum * sizeof(Elf_Shdr) >
> info->len - info->hdr->e_shoff))
> return -ENOEXEC;
>
>+ info->sechdrs = (void *)info->hdr + info->hdr->e_shoff;
>+
>+ /*
>+ * Verify if the section name table index is valid.
>+ */
>+ if (info->hdr->e_shstrndx == SHN_UNDEF
>+ || info->hdr->e_shstrndx >= info->hdr->e_shnum)
>+ return -ENOEXEC;
>+
>+ strhdr = &info->sechdrs[info->hdr->e_shstrndx];
>+ err = validate_section_offset(info, strhdr);
>+ if (err < 0)
>+ return err;
>+
>+ /*
>+ * The section name table must be NUL-terminated, as required
>+ * by the spec. This makes strcmp and pr_* calls that access
>+ * strings in the section safe.
>+ */
>+ info->secstrings = (void *)info->hdr + strhdr->sh_offset;
>+ if (info->secstrings[strhdr->sh_size - 1] != '\0')
>+ return -ENOEXEC;
>+
>+ /*
>+ * The code assumes that section 0 has a length of zero and
>+ * an addr of zero, so check for it.
>+ */
>+ if (info->sechdrs[0].sh_type != SHT_NULL
>+ || info->sechdrs[0].sh_size != 0
>+ || info->sechdrs[0].sh_addr != 0)
>+ return -ENOEXEC;
>+
>+ for (i = 1; i < info->hdr->e_shnum; i++) {
>+ shdr = &info->sechdrs[i];
>+ switch (shdr->sh_type) {
>+ case SHT_NULL:
>+ case SHT_NOBITS:
>+ continue;
>+ case SHT_SYMTAB:
>+ if (shdr->sh_link == SHN_UNDEF
>+ || shdr->sh_link >= info->hdr->e_shnum)
>+ return -ENOEXEC;
>+ fallthrough;
>+ default:
>+ err = validate_section_offset(info, shdr);
>+ if (err < 0) {
>+ pr_err("invalid ELF section in module num %u type %u\n",
>+ i, shdr->sh_type);

Same as the first comment here. Also, this is personal preference but I
think the "in module num %u type %u" reads a bit awkwardly. Maybe
something like "Invalid ELF section in module (section ndx %u type %u)"?

>+ return err;
>+ }
>+
>+ if (shdr->sh_flags & SHF_ALLOC) {
>+ if (shdr->sh_name >= strhdr->sh_size) {
>+ pr_err("invalid ELF section name in module num %u type %u\n",
>+ i, shdr->sh_type);

Same here.

Thanks!

Jessica

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