Commit message: BUG/CRITICAL: hpack: never index a header into the headroom after wrapping

The HPACK header table is implemented as a wrapping list inside a contigous
area. Headers names and values are stored from right to left while indexes
are stored from left to right. When there's no more room to store a new one,
we wrap to the right again, or possibly defragment it if needed. The condition
do use the right part (called tailroom) or the left part (called headroom)
depends on the location of the last inserted header. After wrapping happens,
the code forces to stick to tailroom by pretending there's no more headroom,
so that the size fit test always fails. The problem is that nothing prevents
from storing a header with an empty name and empty value, resulting in a
total size of zero bytes, which satisfies the condition to use the headroom.
Doing this in a wrapped buffer results in changing the "front" header index
and causing miscalculations on the available size and the addresses of the
next headers. This may even allow to overwrite some parts of the index,
opening the possibility to perform arbitrary writes into a 32-bit relative
address space.

This patch fixes the issue by making sure the headroom is considered only
when the buffer does not wrap, instead of relying on the zero size. This
must be backported to all versions supporting H2, which is as far as 1.8.

Many thanks to Felix Wilhelm of Google Project Zero for responsibly
reporting this problem with a reproducer and a detailed analysis.
CVE-2020-11100 was assigned to this issue.
CVE: CVE-2020-11100
CWE: CWE-787
Vulnerability description: In hpack_dht_insert in hpack-tbl.c in the HPACK decoder in HAProxy 1.8 through 2.x before 2.1.4, a remote attacker can write arbitrary bytes around a certain location on the heap via a crafted HTTP/2 request, possibly causing remote code execution.

Project: haproxy
File: hpack-tbl.c

int hpack_dht_insert(struct hpack_dht *dht, struct ist name, struct ist value)
{
	unsigned int used;
	unsigned int head;
	unsigned int prev;
	unsigned int wrap;
	unsigned int tail;
	uint32_t headroom, tailroom;

	if (!hpack_dht_make_room(dht, name.len + value.len))
		return 0;

	/* Now there is enough room in the table, that's guaranteed by the
	 * protocol, but not necessarily where we need it.
	 */

	used = dht->used;
	if (!used) {
		/* easy, the table was empty */
		dht->front = dht->head = 0;
		dht->wrap  = dht->used = 1;
		dht->total = 0;
		head = 0;
		dht->dte[head].addr = dht->size - (name.len + value.len);
		goto copy;
	}

	/* compute the new head, used and wrap position */
	prev = head = dht->head;
	wrap = dht->wrap;
	tail = hpack_dht_get_tail(dht);

	used++;
	head++;

	if (head >= wrap) {
		/* head is leading the entries, we either need to push the
		 * table further or to loop back to released entries. We could
		 * force to loop back when at least half of the allocatable
		 * entries are free but in practice it never happens.
		 */
		if ((sizeof(*dht) + (wrap + 1) * sizeof(dht->dte[0]) <= dht->dte[dht->front].addr))
			wrap++;
		else if (head >= used) /* there's a hole at the beginning */
			head = 0;
		else {
			/* no more room, head hits tail and the index cannot be
			 * extended, we have to realign the whole table.
			 */
			if (!hpack_dht_defrag(dht))
				return -1;

			wrap = dht->wrap + 1;
			head = dht->head + 1;
			prev = head - 1;
			tail = 0;
		}
	}
	else if (used >= wrap) {
		/* we've hit the tail, we need to reorganize the index so that
		 * the head is at the end (but not necessarily move the data).
		 */
		if (!hpack_dht_defrag(dht))
			return -1;

		wrap = dht->wrap + 1;
		head = dht->head + 1;
		prev = head - 1;
		tail = 0;
	}

	/* Now we have updated head, used and wrap, we know that there is some
	 * available room at least from the protocol's perspective. This space
	 * is split in two areas :
	 *
	 *   1: if the previous head was the front cell, the space between the
	 *      end of the index table and the front cell's address.
	 *   2: if the previous head was the front cell, the space between the
	 *      end of the tail and the end of the table ; or if the previous
	 *      head was not the front cell, the space between the end of the
	 *      tail and the head's address.
	 */
	if (prev == dht->front) {
		/* the area was contiguous */
		headroom = dht->dte[dht->front].addr - (sizeof(*dht) + wrap * sizeof(dht->dte[0]));
		tailroom = dht->size - dht->dte[tail].addr - dht->dte[tail].nlen - dht->dte[tail].vlen;
	}
	else {
		/* it's already wrapped so we can't store anything in the headroom */
		headroom = 0;
		tailroom = dht->dte[prev].addr - dht->dte[tail].addr - dht->dte[tail].nlen - dht->dte[tail].vlen;
	}

	/* We can decide to stop filling the headroom as soon as there's enough
	 * room left in the tail to suit the protocol, but tests show that in
	 * practice it almost never happens in other situations so the extra
	 * test is useless and we simply fill the headroom as long as it's
	 * available.
	 */
	if (headroom >= name.len + value.len) {
		/* install upfront and update ->front */
		dht->dte[head].addr = dht->dte[dht->front].addr - (name.len + value.len);
		dht->front = head;
	}
	else if (tailroom >= name.len + value.len) {
		dht->dte[head].addr = dht->dte[tail].addr + dht->dte[tail].nlen + dht->dte[tail].vlen + tailroom - (name.len + value.len);
	}
	else {
		/* need to defragment the table before inserting upfront */
		dht = hpack_dht_defrag(dht);
		wrap = dht->wrap + 1;
		head = dht->head + 1;
		dht->dte[head].addr = dht->dte[dht->front].addr - (name.len + value.len);
		dht->front = head;
	}

	dht->wrap = wrap;
	dht->head = head;
	dht->used = used;

 copy:
	dht->total         += name.len + value.len;
	dht->dte[head].nlen = name.len;
	dht->dte[head].vlen = value.len;

	memcpy((void *)dht + dht->dte[head].addr, name.ptr, name.len);
	memcpy((void *)dht + dht->dte[head].addr + name.len, value.ptr, value.len);
	return 0;
}