Re: [PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[Simon Horman <horms@kernel.org>]

On Mon, Aug 21, 2023 at 09:48:48PM +0200, Sven Eckelmann wrote:
> The automatic recalculation of the maximum allowed MTU is usually triggered
> by code sections which are already rtnl lock protected by callers outside
> of batman-adv. But when the fragmentation setting is changed via
> batman-adv's own batadv genl family, then the rtnl lock is not yet taken.
> 
> But dev_set_mtu requires that the caller holds the rtnl lock because it
> uses netdevice notifiers. And this code will then fail the check for this
> lock:
> 
>   RTNL: assertion failed at net/core/dev.c (1953)
> 
> Cc: stable@vger.kernel.org
> Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com
> Fixes: c6a953cce8d0 ("batman-adv: Trigger events for auto adjusted MTU")
> Signed-off-by: Sven Eckelmann <sven@narfation.org>

Reviewed-by: Simon Horman <horms@kernel.org>

> ---
> This problem was just identified by syzbot [1]. I hope it is ok to directly
> send this patch to netdev instead of creating a single-patch PR from
> the batadv/net branch. If you still prefer a PR then we can also prepare
> it.
> 
> [1] https://lore.kernel.org/r/0000000000009bbb4b0603717cde@google.com

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