From 4e372dc350be5c72b88546bf03392a5793cea179 Mon Sep 17 00:00:00 2001
From: Willy Tarreau <w@1wt.eu>
Date: Sun, 29 Mar 2020 08:53:31 +0200
Subject: 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.
---
 src/hpack-tbl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/hpack-tbl.c b/src/hpack-tbl.c
index 70d7f35834..727ff7a17b 100644
--- a/src/hpack-tbl.c
+++ b/src/hpack-tbl.c
@@ -346,9 +346,9 @@ int hpack_dht_insert(struct hpack_dht *dht, struct ist name, struct ist value)
 	 * 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.
+	 * available and we don't wrap.
 	 */
-	if (headroom >= name.len + value.len) {
+	if (prev == dht->front && 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;
-- 
2.20.1