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SubjectRe: [PATCH 05/10] x86/cet: ELF header parsing of Control Flow Enforcement
On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> Look in .note.gnu.property of an ELF file and check if shadow stack needs
> to be enabled for the task.

Nice! But please structure it so it's one function that parses out
all the ELF notes and some other code (a table or a switch statement)
that handles them. We will probably want to add more kernel-parsed
ELF notes some day, so let's structure the code to make it easier.

> +static int find_cet(u8 *buf, u32 size, u32 align, int *shstk, int *ibt)
> +{
> + unsigned long start = (unsigned long)buf;
> + struct elf_note *note = (struct elf_note *)buf;
> +
> + *shstk = 0;
> + *ibt = 0;
> +
> + /*
> + * Go through the x86_note_gnu_property array pointed by
> + * buf and look for shadow stack and indirect branch
> + * tracking features.
> + * The GNU_PROPERTY_X86_FEATURE_1_AND entry contains only
> + * one u32 as data. Do not go beyond buf_size.
> + */
> +
> + while ((unsigned long) (note + 1) - start < size) {
> + /* Find the NT_GNU_PROPERTY_TYPE_0 note. */
> + if (note->n_namesz == 4 &&
> + note->n_type == NT_GNU_PROPERTY_TYPE_0 &&
> + memcmp(note + 1, "GNU", 4) == 0) {
> + u8 *ptr, *ptr_end;
> +
> + /* Check for invalid property. */
> + if (note->n_descsz < 8 ||
> + (note->n_descsz % align) != 0)
> + return 0;
> +
> + /* Start and end of property array. */
> + ptr = (u8 *)(note + 1) + 4;
> + ptr_end = ptr + note->n_descsz;

Exploitable bug here? You haven't checked that ptr is in bounds or
that ptr + ptr_end is in bounds (or that ptr_end > ptr, for that
matter).

> +
> + while (1) {
> + u32 type = *(u32 *)ptr;
> + u32 datasz = *(u32 *)(ptr + 4);
> +
> + ptr += 8;
> + if ((ptr + datasz) > ptr_end)
> + break;
> +
> + if (type == GNU_PROPERTY_X86_FEATURE_1_AND &&
> + datasz == 4) {
> + u32 p = *(u32 *)ptr;
> +
> + if (p & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
> + *shstk = 1;
> + if (p & GNU_PROPERTY_X86_FEATURE_1_IBT)
> + *ibt = 1;
> + return 1;
> + }
> + }
> + }
> +
> + /*
> + * Note sections like .note.ABI-tag and .note.gnu.build-id
> + * are aligned to 4 bytes in 64-bit ELF objects.
> + */
> + note = (void *)note + ELF_NOTE_NEXT_OFFSET(note, align);

A malicious value here will probably just break out of the while
statement, but it's still scary.

> + }
> +
> + return 0;
> +}
> +
> +static int check_pt_note_segment(struct file *file,
> + unsigned long note_size, loff_t *pos,
> + u32 align, int *shstk, int *ibt)
> +{
> + int retval;
> + char *note_buf;
> +
> + /*
> + * Try to read in the whole PT_NOTE segment.
> + */
> + note_buf = kmalloc(note_size, GFP_KERNEL);

kmalloc() with fully user-controlled, unchecked size is not a good idea.

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