netdev - [PATCH] bpf: Fix integer overflow involving bucket

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Message-Id: <20210805140515.35630-1-th.yasumatsu@gmail.com>
Date:   Thu,  5 Aug 2021 23:05:15 +0900
From:   Tatsuhiko Yasumatsu <th.yasumatsu@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     th.yasumatsu@...il.com, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] bpf: Fix integer overflow involving bucket_size

In __htab_map_lookup_and_delete_batch(), hash buckets are iterated over
to count the number of elements in each bucket (bucket_size).
If bucket_size is large enough, the multiplication to calculate
kvmalloc() size could overflow, resulting in out-of-bounds write
as reported by KASAN.

[...]
[  104.986052] BUG: KASAN: vmalloc-out-of-bounds in __htab_map_lookup_and_delete_batch+0x5ce/0xb60
[  104.986489] Write of size 4194224 at addr ffffc9010503be70 by task crash/112
[  104.986889]
[  104.987193] CPU: 0 PID: 112 Comm: crash Not tainted 5.14.0-rc4 #13
[  104.987552] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
[  104.988104] Call Trace:
[  104.988410]  dump_stack_lvl+0x34/0x44
[  104.988706]  print_address_description.constprop.0+0x21/0x140
[  104.988991]  ? __htab_map_lookup_and_delete_batch+0x5ce/0xb60
[  104.989327]  ? __htab_map_lookup_and_delete_batch+0x5ce/0xb60
[  104.989622]  kasan_report.cold+0x7f/0x11b
[  104.989881]  ? __htab_map_lookup_and_delete_batch+0x5ce/0xb60
[  104.990239]  kasan_check_range+0x17c/0x1e0
[  104.990467]  memcpy+0x39/0x60
[  104.990670]  __htab_map_lookup_and_delete_batch+0x5ce/0xb60
[  104.990982]  ? __wake_up_common+0x4d/0x230
[  104.991256]  ? htab_of_map_free+0x130/0x130
[  104.991541]  bpf_map_do_batch+0x1fb/0x220
[...]

In hashtable, if the elements' keys have the same jhash() value, the
elements will be put into the same bucket. By putting a lot of elements
into a single bucket, the value of bucket_size can be increased to
trigger the integer overflow.

Triggering the overflow is possible for both callers with CAP_SYS_ADMIN
and callers without CAP_SYS_ADMIN.

It will be trivial for a caller with CAP_SYS_ADMIN to intentionally
reach this overflow by enabling BPF_F_ZERO_SEED. As this flag will set
the random seed passed to jhash() to 0, it will be easy for the caller
to prepare keys which will be hashed into the same value, and thus put
all the elements into the same bucket.

If the caller does not have CAP_SYS_ADMIN, BPF_F_ZERO_SEED cannot be
used. However, it will be still technically possible to trigger the
overflow, by guessing the random seed value passed to jhash() (32bit)
and repeating the attempt to trigger the overflow. In this case,
the probability to trigger the overflow will be low and will take
a very long time.

Fix the integer overflow by casting 1 operand to u64.

Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
Signed-off-by: Tatsuhiko Yasumatsu <th.yasumatsu@...il.com>
---
 kernel/bpf/hashtab.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 72c58cc516a3..e29283c3b17f 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1565,8 +1565,8 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
 	/* We cannot do copy_from_user or copy_to_user inside
 	 * the rcu_read_lock. Allocate enough space here.
 	 */
-	keys = kvmalloc(key_size * bucket_size, GFP_USER | __GFP_NOWARN);
-	values = kvmalloc(value_size * bucket_size, GFP_USER | __GFP_NOWARN);
+	keys = kvmalloc((u64)key_size * bucket_size, GFP_USER | __GFP_NOWARN);
+	values = kvmalloc((u64)value_size * bucket_size, GFP_USER | __GFP_NOWARN);
 	if (!keys || !values) {
 		ret = -ENOMEM;
 		goto after_loop;
-- 
2.25.1