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Re: [Qemu-devel] [PATCH for-2.9?] block: Do not unref bs->file on error

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH for-2.9?] block: Do not unref bs->file on error in BD's open
Date: Thu, 13 Apr 2017 11:21:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 04/13/2017 10:43 AM, Max Reitz wrote:
> The block layer takes care of removing the bs->file child if the block
> driver's bdrv_open()/bdrv_file_open() implementation fails. The block
> driver therefore does not need to do so, and indeed should not unless it
> sets bs->file to NULL afterwards -- because if this is not done, the
> bdrv_unref_child() in bdrv_open_inherit() will dereference the freed
> memory block at bs->file afterwards, which is not good.
> We can now decide whether to add a "bs->file = NULL;" after each of the
> offending bdrv_unref_child() invocations, or just drop them altogether.
> The latter is simpler, so let's do that.
> Cc: qemu-stable <address@hidden>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> It's an issue only in blkdebug, blkreplace and blkverify, and only when
> an error occurs in their open functions; therefore I think this is fine
> to delay until 2.10.
> However, it *is* a use-after-free newly introduced in 2.9, so that's
> where the question mark comes from...
> ---
>  block/blkdebug.c  | 4 +---
>  block/blkreplay.c | 3 ---
>  block/blkverify.c | 3 ---
>  3 files changed, 1 insertion(+), 9 deletions(-)

On its own, this patch is not worthy of an -rc5 (it only affects failure
paths, and 2 of the 3 affected drivers are not for production use). But
use-after-free is annoying, and the fact that blkreplay is affected
could trip up a user that mistypes arguments causing a BD open to fail.
I'd argue that since we're looking at -rc5 anyways, this is fair game
for inclusion in that build, since it is a small enough patch and easy
to review for correctness.  Hence I changed the subject in my reply to

But I won't lose any sleep if we just wait for 2.9.1 and 2.10.

Reviewed-by: Eric Blake <address@hidden>

> diff --git a/block/blkdebug.c b/block/blkdebug.c
> index 67e8024e36..cc4a146e84 100644
> --- a/block/blkdebug.c
> +++ b/block/blkdebug.c
> @@ -389,14 +389,12 @@ static int blkdebug_open(BlockDriverState *bs, QDict 
> *options, int flags,
>      } else if (align) {
>          error_setg(errp, "Invalid alignment");
>          ret = -EINVAL;
> -        goto fail_unref;
> +        goto out;
>      }

And this means I get to rebase my blkdebug patches on top of yours, if
yours goes in first.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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