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Re: [Qemu-devel] [PATCH 09/14] block-backend: Fix potential double blk_d

From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH 09/14] block-backend: Fix potential double blk_delete()
Date: Tue, 11 Sep 2018 16:27:46 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Fri, 09/07 18:15, Kevin Wolf wrote:
> blk_unref() first decreases the refcount of the BlockBackend and calls
> blk_delete() if the refcount reaches zero. Requests can still be in
> flight at this point, they are only drained during blk_delete():
> At this point, arbitrary callbacks can run. If any callback takes a
> temporary BlockBackend reference, it will first increase the refcount to
> 1 and then decrease it to 0 again, triggering another blk_delete(). This
> will cause a use-after-free crash in the outer blk_delete().
> Fix it by draining the BlockBackend before decreasing to refcount to 0.
> Assert in blk_ref() that it never takes the first refcount (which would
> mean that the BlockBackend is already being deleted).
> Signed-off-by: Kevin Wolf <address@hidden>

Good one!

Reviewed-by: Fam Zheng <address@hidden>

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