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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>
Subject: [PATCH 5.12 672/677] bpf, ringbuf: Deny reserve of buffers larger than ringbuf
Date:   Wed, 12 May 2021 16:51:58 +0200
Message-Id: <20210512144859.685292990@linuxfoundation.org>
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From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

commit 4b81ccebaeee885ab1aa1438133f2991e3a2b6ea upstream.

A BPF program might try to reserve a buffer larger than the ringbuf size.
If the consumer pointer is way ahead of the producer, that would be
successfully reserved, allowing the BPF program to read or write out of
the ringbuf allocated area.

Reported-by: Ryota Shiga (Flatt Security)
Fixes: 457f44363a88 ("bpf: Implement BPF ring buffer and verifier support for it")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/bpf/ringbuf.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/bpf/ringbuf.c
+++ b/kernel/bpf/ringbuf.c
@@ -315,6 +315,9 @@ static void *__bpf_ringbuf_reserve(struc
 		return NULL;
 
 	len = round_up(size + BPF_RINGBUF_HDR_SZ, 8);
+	if (len > rb->mask + 1)
+		return NULL;
+
 	cons_pos = smp_load_acquire(&rb->consumer_pos);
 
 	if (in_nmi()) {