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From: Bui Quang Minh <minhquangbui99@gmail.com>
To: Lorenz Bauer <lmb@cloudflare.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	hawk@kernel.org, John Fastabend <john.fastabend@gmail.com>,
	Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	kpsingh@kernel.org, Jakub Sitnicki <jakub@cloudflare.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] bpf: Fix integer overflow in argument calculation for bpf_map_area_alloc
Date: Wed, 27 Jan 2021 11:23:41 +0700	[thread overview]
Message-ID: <20210127042341.GA4948@ubuntu> (raw)
In-Reply-To: <CACAyw99bEYWJCSGqfLiJ9Jp5YE1ZsZSiJxb4RFUTwbofipf0dA@mail.gmail.com>

On Tue, Jan 26, 2021 at 09:36:57AM +0000, Lorenz Bauer wrote:
> On Tue, 26 Jan 2021 at 08:26, Bui Quang Minh <minhquangbui99@gmail.com> wrote:
> >
> > In 32-bit architecture, the result of sizeof() is a 32-bit integer so
> > the expression becomes the multiplication between 2 32-bit integer which
> > can potentially leads to integer overflow. As a result,
> > bpf_map_area_alloc() allocates less memory than needed.
> >
> > Fix this by casting 1 operand to u64.
> Some quick thoughts:
> * Should this have a Fixes tag?

Ok, I will add Fixes tag in later version patch.

> * Seems like there are quite a few similar calls scattered around
> (cpumap, etc.). Did you audit these as well?

I spotted another bug after re-auditting. In hashtab, there ares 2 places using
the same calls

	static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
		/* ... snip ... */
		if (htab->n_buckets == 0 ||
		    htab->n_buckets > U32_MAX / sizeof(struct bucket))
			goto free_htab;

		htab->buckets = bpf_map_area_alloc(htab->n_buckets *
						   sizeof(struct bucket),

This is safe because of the above check.

	static int prealloc_init(struct bpf_htab *htab)
		u32 num_entries = htab->map.max_entries;
		htab->elems = bpf_map_area_alloc(htab->elem_size * num_entries,

This is not safe since there is no limit check in elem_size.

In cpumap,

	static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
		cmap->cpu_map = bpf_map_area_alloc(cmap->map.max_entries *
						   sizeof(struct bpf_cpu_map_entry *),

I think this is safe because max_entries is not permitted to be larger than NR_CPUS.

In stackmap, there is a place that I'm not very sure about

	static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
		u32 elem_size = sizeof(struct stack_map_bucket) + smap->map.value_size;
		smap->elems = bpf_map_area_alloc(elem_size * smap->map.max_entries,

This is called after another bpf_map_area_alloc in stack_map_alloc(). In the first
bpf_map_area_alloc() the argument is calculated in an u64 variable; so if in the second
one, there is an integer overflow then the first one must be called with size > 4GB. I 
think the first one will probably fail (I am not sure about the actual limit of vmalloc()),
so the second one might not be called.

Overall, I think it is error prone in this pattern, maybe we should use typecasting in all
similar calls or make a comment why we don't use typecasting. As I see typecasting is not so
expensive and we can typecast the sizeof() operand so this change only affect 32-bit

> * I'd prefer a calloc style version of bpf_map_area_alloc although
> that might conflict with Fixes tag.

Yes, I think the calloc style will prevent this kind of integer overflow bug.

Thank you,
Quang Minh.

  reply	other threads:[~2021-01-27  5:47 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-26  8:26 [PATCH] bpf: Fix integer overflow in argument calculation for bpf_map_area_alloc Bui Quang Minh
2021-01-26  9:36 ` Lorenz Bauer
2021-01-27  4:23   ` Bui Quang Minh [this message]
2021-01-27  5:09     ` Bui Quang Minh
2021-01-28  0:41     ` Daniel Borkmann
2021-05-17 15:10       ` Bui Quang Minh

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