netdev - Re: [PATCH net] bpf: fix allocation warnings in bpf maps and integer overflow

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Date:	Mon, 30 Nov 2015 10:13:11 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	davem@...emloft.net, Alexei Starovoitov <ast@...nel.org>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH net] bpf: fix allocation warnings in bpf maps and integer
 overflow

On Mon, Nov 30, 2015 at 03:34:35PM +0100, Daniel Borkmann wrote:
> >>diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> >>index 3f4c99e06c6b..b1e53b79c586 100644
> >>--- a/kernel/bpf/arraymap.c
> >>+++ b/kernel/bpf/arraymap.c
> >>@@ -28,11 +28,17 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
> >>          attr->value_size == 0)
> >>          return ERR_PTR(-EINVAL);
> >>
> >>+    if (attr->value_size >= 1 << (KMALLOC_SHIFT_MAX - 1))
> >>+        /* if value_size is bigger, the user space won't be able to
> >>+         * access the elements.
> >>+         */
> >>+        return ERR_PTR(-E2BIG);
> >>+
> >
> >Bit confused, given that in array map, we try kzalloc() with __GFP_NOWARN already
> >and if that fails, we fall back to vzalloc(), it shouldn't trigger memory allocation
> >warnings here ...

not quite, the above check is for kmalloc-s in syscall.c

> Ok, I see. The check and comment is related to the fact that when we do bpf(2)
> syscall to lookup an element:
> 
> We call map_lookup_elem(), which does kmalloc() on the value_size.
> 
> So an individual entry lookup could fail with kmalloc() there, unrelated to an
> individual map implementation.

kmalloc with order >= MAX_ORDER warning can be seen in syscall for update/lookup
commands regardless of map implememtation.
So the maps with "value_size >= 1 << (KMALLOC_SHIFT_MAX - 1)" were not accessible
from user space anyway.
This check in arraymap.c fixes the warning and prevents creation of such
maps in the first place as the comment right below it says.
Similar check in hashmap.c fixes warning, prevents abnormal map creation and fixes
integer overflow which is the most dangerous of them all.

The check in arraymap.c
-        attr->max_entries > (U32_MAX - sizeof(*array)) / elem_size)
+        attr->max_entries > (U32_MAX - PAGE_SIZE - sizeof(*array)) / elem_size)
 fixes potential integer overflow in map.pages computation.

and similar check in hashtab.c:
(u64) htab->elem_size * htab->map.max_entries >= U32_MAX - PAGE_SIZE
fixes integer overflow in map.pages as well.

the 'value_size >= (1 << (KMALLOC_SHIFT_MAX - 1)) - MAX_BPF_STACK - sizeof(struct htab_elem)'
check in hashmap.c fixes integer overflow in elem_size and
makes elem_size kmalloc-able later in htab_map_update_elem().
Since it wasn't obvious that this one 'if' addresses these multiple issues,
I've added a comment there.

Addition of __GFP_NOWARN only fixes OOM warning as commit log says.

> Hmm, seems this patch fixes many things at once, maybe makes sense to split it?

hmm I don't see a point of changing the same single line over multipe patches.
The split won't help backporting, but rather makes for more patches to deal with.

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