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

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Message-ID: <CANn89iJY4=Q0edL-mf2JrRiz8Ld7bQcogOrc4ozLEVD8qz8o2A@mail.gmail.com>
Date: Mon, 4 Sep 2023 11:06:17 +0200
From: Eric Dumazet <edumazet@...gle.com>
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" <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()

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()

I personally can not test 32bit kernels.