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libkmod: properly validate file size

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In function kmod_elf_new, the file size has to be properly validated against
section offset. Currently, the file size is considered valid based on
ELF header size + section header size * section count. That is not sufficient.

In fact, ELF specifies a section header offset, which doesn't have to be the
size of the ELF header. The supplied test cases even cover this.

The correct test is: section offset + section header size * section count

This patch also verifies that this value won't overflow.  I don't know a way
to crash a tool due to this bug, because later on the offset check would
prevent out-of-bounds access. An overflow would just mean to access a wrong
part in elf->memory. Yet it's a validation error.

Please note: The file size does not have to be validated against the size
of the ELF header again, elf_identify did this already.
This commit is contained in:
Tobias Stoeckmann 2015-02-10 19:46:40 +01:00 committed by Lucas De Marchi
parent 246d67d493
commit 249dc5909b
1 changed files with 5 additions and 5 deletions
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10
libkmod/libkmod-elf.c
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@ -272,7 +272,8 @@ static const char *elf_get_strings_section(const struct kmod_elf *elf, uint64_t
struct kmod_elf *kmod_elf_new(const void *memory, off_t size)
{
struct kmod_elf *elf;
size_t hdr_size, shdr_size, min_size;
uint64_t min_size;
size_t shdrs_size, shdr_size;
int class;
assert_cc(sizeof(uint16_t) == sizeof(Elf32_Half));
@ -308,12 +309,10 @@ struct kmod_elf *kmod_elf_new(const void *memory, off_t size)
if (elf->class & KMOD_ELF_32) {
const Elf32_Ehdr *hdr _unused_ = elf_get_mem(elf, 0);
LOAD_HEADER;
hdr_size = sizeof(Elf32_Ehdr);
shdr_size = sizeof(Elf32_Shdr);
} else {
const Elf64_Ehdr *hdr _unused_ = elf_get_mem(elf, 0);
LOAD_HEADER;
hdr_size = sizeof(Elf64_Ehdr);
shdr_size = sizeof(Elf64_Shdr);
}
#undef LOAD_HEADER
@ -330,8 +329,9 @@ struct kmod_elf *kmod_elf_new(const void *memory, off_t size)
elf->header.section.entry_size, shdr_size);
goto invalid;
}
min_size = hdr_size + shdr_size * elf->header.section.count;
if (min_size >= elf->size) {
shdrs_size = shdr_size * elf->header.section.count;
if (addu64_overflow(shdrs_size, elf->header.section.offset, &min_size)
|| min_size > elf->size) {
ELFDBG(elf, "file is too short to hold sections\n");
goto invalid;
}