netdev - RE: [PATCH net] net: deal with integer overflows in kmalloc

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Date: Mon, 4 Sep 2023 09:27:28 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Eric Dumazet' <edumazet@...gle.com>
CC: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	syzbot <syzkaller@...glegroups.com>, Kyle Zeng <zengyhkyle@...il.com>, "Kees
 Cook" <keescook@...omium.org>, Vlastimil Babka <vbabka@...e.cz>
Subject: RE: [PATCH net] net: deal with integer overflows in kmalloc_reserve()

From: Eric Dumazet <edumazet@...gle.com>
> Sent: 04 September 2023 10:06
> To: David Laight <David.Laight@...LAB.COM>
> Cc: David S . Miller <davem@...emloft.net>; Jakub Kicinski <kuba@...nel.org>; Paolo Abeni
> <pabeni@...hat.com>; netdev@...r.kernel.org; eric.dumazet@...il.com; syzbot
> <syzkaller@...glegroups.com>; Kyle Zeng <zengyhkyle@...il.com>; Kees Cook <keescook@...omium.org>;
> Vlastimil Babka <vbabka@...e.cz>
> Subject: Re: [PATCH net] net: deal with integer overflows in kmalloc_reserve()
> 
> On Mon, Sep 4, 2023 at 10:41 AM David Laight <David.Laight@...lab.com> wrote:
> >
> > From: Eric Dumazet
> > > Sent: 31 August 2023 19:38
> > >
> > > Blamed commit changed:
> > >     ptr = kmalloc(size);
> > >     if (ptr)
> > >       size = ksize(ptr);
> > >
> > > to:
> > >     size = kmalloc_size_roundup(size);
> > >     ptr = kmalloc(size);
> > >
> > > This allowed various crash as reported by syzbot [1]
> > > and Kyle Zeng.
> > >
> > > Problem is that if @size is bigger than 0x80000001,
> > > kmalloc_size_roundup(size) returns 2^32.
> > >
> > > kmalloc_reserve() uses a 32bit variable (obj_size),
> > > so 2^32 is truncated to 0.
> >
> > Can this happen on 32bit arch?
> > In that case kmalloc_size_roundup() will return 0.
> 
> Maybe, but this would be a bug in kmalloc_size_roundup()

That contains:
	/* Short-circuit saturated "too-large" case. */
	if (unlikely(size == SIZE_MAX))
		return SIZE_MAX;

It can also return 0 on failure, I can't remember if kmalloc(0)
is guaranteed to be NULL (malloc(0) can do 'other things').

Which is entirely hopeless since MAX_SIZE is (size_t)-1.

IIRC kmalloc() has a size limit (max 'order' of pages) so
kmalloc_size_roundup() ought check for that (or its max value).

The final:
	/* The flags don't matter since size_index is common to all. */
	c = kmalloc_slab(size, GFP_KERNEL);
	return c ? c->object_size : 0;
probably ought to return size if c is even NULL.

	David

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