lkml.org 
[lkml]   [2021]   [Jan]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH v2] module: harden ELF info handling
+++ Frank van der Linden [07/01/21 19:30 +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.
>
>- 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..34fc6c85eb65 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);
> 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)
>+ 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 (section %u type %u)\n",
>+ i, shdr->sh_type);
>+ return err;
>+ }
>+
>+ if (shdr->sh_flags & SHF_ALLOC) {
>+ if (shdr->sh_name >= strhdr->sh_size) {
>+ pr_err("Invalid ELF section name in module (section num %u type %u)\n",

Small nit: Maybe remove "num", to be consistent with the other pr_err() above.

>+ i, shdr->sh_type);
>+ return -ENOEXEC;
>+ }
>+ }
>+ break;
>+ }
>+ }
>+
> return 0;
> }
>
>@@ -3095,11 +3186,6 @@ static int rewrite_section_headers(struct load_info *info, int flags)
>
> for (i = 1; i < info->hdr->e_shnum; i++) {
> Elf_Shdr *shdr = &info->sechdrs[i];
>- if (shdr->sh_type != SHT_NOBITS
>- && info->len < shdr->sh_offset + shdr->sh_size) {
>- pr_err("Module len %lu truncated\n", info->len);
>- return -ENOEXEC;
>- }
>
> /*
> * Mark all sections sh_addr with their address in the
>@@ -3133,11 +3219,6 @@ static int setup_load_info(struct load_info *info, int flags)
> {
> unsigned int i;
>
>- /* Set up the convenience variables */
>- info->sechdrs = (void *)info->hdr + info->hdr->e_shoff;
>- info->secstrings = (void *)info->hdr
>- + info->sechdrs[info->hdr->e_shstrndx].sh_offset;
>-
> /* Try to find a name early so we can log errors with a module name */
> info->index.info = find_sec(info, ".modinfo");
> if (info->index.info)
>@@ -3894,26 +3975,50 @@ static int load_module(struct load_info *info, const char __user *uargs,
> long err = 0;
> char *after_dashes;
>
>- err = elf_header_check(info);
>+ /*
>+ * Do the signature check (if any) first. All that
>+ * the signature check needs is info->len, it does
>+ * not need any of the section info. That can be
>+ * set up later. This will minimize the chances
>+ * of a corrupt module causing problems before
>+ * we even get to the signature check.
>+ *
>+ * The check will also adjust info->len by stripping
>+ * off the sig length at the end of the module, making
>+ * checks against info->len more correct.
>+ */
>+ err = module_sig_check(info, flags);
>+ if (err)
>+ goto free_copy;
>+
>+ /*
>+ * Do basic sanity checks against the ELF header and
>+ * sections.
>+ */
>+ err = elf_validity_check(info);
> if (err) {
>- pr_err("Module has invalid ELF header\n");
>+ pr_err("Module has invalid ELF structures\n");
> goto free_copy;
> }
>
>+ /*
>+ * Everything checks out, so set up the section info
>+ * in the info structure.
>+ */
> err = setup_load_info(info, flags);
> if (err)
> goto free_copy;
>
>+ /*
>+ * Now that we know we have the correct module name, check
>+ * if it's blacklisted.
>+ */
> if (blacklisted(info->name)) {
> err = -EPERM;
> pr_err("Module %s is blacklisted\n", info->name);
> goto free_copy;
> }
>
>- err = module_sig_check(info, flags);
>- if (err)
>- goto free_copy;
>-
> err = rewrite_section_headers(info, flags);
> if (err)
> goto free_copy;
>diff --git a/kernel/module_signature.c b/kernel/module_signature.c
>index 4224a1086b7d..00132d12487c 100644
>--- a/kernel/module_signature.c
>+++ b/kernel/module_signature.c
>@@ -25,7 +25,7 @@ int mod_check_sig(const struct module_signature *ms, size_t file_len,
> return -EBADMSG;
>
> if (ms->id_type != PKEY_ID_PKCS7) {
>- pr_err("%s: Module is not signed with expected PKCS#7 message\n",
>+ pr_err("%s: not signed with expected PKCS#7 message\n",
> name);
> return -ENOPKG;
> }
>diff --git a/kernel/module_signing.c b/kernel/module_signing.c
>index 9d9fc678c91d..9a057c5d1d4d 100644
>--- a/kernel/module_signing.c
>+++ b/kernel/module_signing.c
>@@ -30,7 +30,7 @@ int mod_verify_sig(const void *mod, struct load_info *info)
>
> memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
>
>- ret = mod_check_sig(&ms, modlen, info->name);
>+ ret = mod_check_sig(&ms, modlen, info->name ?: "module");

Since info->name is not expected to be valid anymore, as we're back to
calling mod_sig_check() first thing, perhaps just stick with
"module"? Or was there another reason for checking info->name here?

Thanks,

Jessica

\
 
 \ /
  Last update: 2021-01-13 15:51    [W:0.034 / U:1.700 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site