Re: [PATCH 4.19 01/51] ethtool: fix potential userspace buffer overflow

[Michal Kubecek]

On Mon, Jun 10, 2019 at 10:21:12AM +0200, Pavel Machek wrote:
> Hi!
> 
> > From: Vivien Didelot <vivien.didelot@xxxxxxxxx>
> > 
> > [ Upstream commit 0ee4e76937d69128a6a66861ba393ebdc2ffc8a2 ]
> > 
> > ethtool_get_regs() allocates a buffer of size ops->get_regs_len(),
> > and pass it to the kernel driver via ops->get_regs() for filling.
> > 
> > There is no restriction about what the kernel drivers can or cannot do
> > with the open ethtool_regs structure. They usually set regs->version
> > and ignore regs->len or set it to the same size as ops->get_regs_len().
> > 
> > But if userspace allocates a smaller buffer for the registers dump,
> > we would cause a userspace buffer overflow in the final copy_to_user()
> > call, which uses the regs.len value potentially reset by the driver.
> > 
> > To fix this, make this case obvious and store regs.len before calling
> > ops->get_regs(), to only copy as much data as requested by userspace,
> > up to the value returned by ops->get_regs_len().
> > 
> > While at it, remove the redundant check for non-null regbuf.
> 
> Mainline differs from 4.19-stable here, and while the non-null check
> is redundant in -mainline, it does not seem to be redundant in
> -stable.
> 
> In stable, if get_regs_len() returns < 0, we'll pass it to vzalloc.

Right. :-(

I guess we should also pick commit f9fc54d313fa ("ethtool: check the
return value of get_regs_len") to stable branches before 5.0.

Michal