Re: [PATCH v4] IB/sa: Resolving use-after-free in ib_nl_send_msg

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Aug 23, 2021 at 04:54:16PM +0000, Haakon Bugge wrote:
> 
> 
> > On 8 Jul 2020, at 03:12, Jason Gunthorpe <jgg@nvidia.com> wrote:
> > 
> > On Tue, Jul 07, 2020 at 06:05:02PM -0700, Divya Indi wrote:
> >> Thanks Jason.
> >> 
> >> Appreciate your help and feedback for fixing this issue.
> >> 
> >> Would it be possible to access the edited version of the patch?
> >> If yes, please share a pointer to the same.
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma.git/commit/?h=for-rc&id=f427f4d6214c183c474eeb46212d38e6c7223d6a
> 
> Hi Jason,
> 
> 
> At first glanse, this commit calls rdma_nl_multicast() whilst
> holding a spinlock. Since rdma_nl_multicast() is called with a
> gfp_flag parameter, one could assume it supports an atomic
> context. rdma_nl_multicast() ends up in
> netlink_broadcast_filtered(). This function calls
> netlink_lock_table(), which calls read_unlock_irqrestore(), which
> ends up calling _raw_read_unlock_irqrestore(). And here
> preempt_enable() is called :-(

I don't understand. This:

	unsigned long flags;

	read_lock_irqsave(&nl_table_lock, flags);
	atomic_inc(&nl_table_users);
	read_unlock_irqrestore(&nl_table_lock, flags);

Is perfectly fine in an atomic context.

preempt_enable is implemented as a nesting counter, so it is fine to
call it from inside an atomic region so long as it is balanced.

Jason