Messages in this thread Patch in this message | | | Date | Tue, 5 Jan 2021 14:39:28 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH] module: harden ELF info handling |
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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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