Re: [PATCH] nvme: prevent double free in nvme_alloc_ns() error handling

Javier González
2020-04-28T07:06:56Z

On 27.04.2020 18:22, Niklas Cassel wrote:
>On Mon, Apr 27, 2020 at 08:03:11PM +0200, Javier González wrote:
>> On 27.04.2020 14:34, Niklas Cassel wrote:
>> > When jumping to the out_put_disk label, we will call put_disk(), which will
>> > trigger a call to disk_release(), which calls blk_put_queue().
>> >
>> > Later in the cleanup code, we do blk_cleanup_queue(), which will also call
>> > blk_put_queue().
>> >
>> > Putting the queue twice is incorrect, and will generate a KASAN splat.
>> >
>> > Set the disk->queue pointer to NULL, before calling put_disk(), so that the
>> > first call to blk_put_queue() will not free the queue.
>> >
>> > The second call to blk_put_queue() uses another pointer to the same queue,
>> > so this call will still free the queue.
>> >
>> > Fixes: 85136c010285 ("lightnvm: simplify geometry enumeration")
>> > Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
>> > ---
>> > drivers/nvme/host/core.c | 2 ++
>> > 1 file changed, 2 insertions(+)
>> >
>> > diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>> > index 91c1bd659947..f2adea96b04c 100644
>> > --- a/drivers/nvme/host/core.c
>> > +++ b/drivers/nvme/host/core.c
>> > @@ -3642,6 +3642,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
>> >
>> > return;
>> > out_put_disk:
>> > + /* prevent double queue cleanup */
>> > + ns->disk->queue = NULL;
>> > put_disk(ns->disk);
>> > out_unlink_ns:
>> > mutex_lock(&ctrl->subsys->lock);
>> > --
>> > 2.25.3
>> >
>> What about delaying the assignment of ns->disk?
>>
>> diff --git i/drivers/nvme/host/core.c w/drivers/nvme/host/core.c
>> index a4d8c90ee7cc..6da4a9ced945 100644
>> --- i/drivers/nvme/host/core.c
>> +++ w/drivers/nvme/host/core.c
>> @@ -3541,7 +3541,6 @@ static int nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
>> disk->queue = ns->queue;
>> disk->flags = flags;
>> memcpy(disk->disk_name, disk_name, DISK_NAME_LEN);
>> - ns->disk = disk;
>>
>> __nvme_revalidate_disk(disk, id);
>>
>> @@ -3553,6 +3552,8 @@ static int nvme_alloc_ns(struct nvme_ctrl *ctrl, unsigned nsid)
>> }
>> }
>>
>> + ns->disk = disk;
>> +
>
>Hello Javier!
>
>
>The only case where we jump to the out_put_disk label, is if the
>nvme_nvm_register() call failed.
>
>In that case, we want to undo the alloc_disk_node() operation, i.e.,
>decrease the refcount.
>
>If we don't set "ns->disk = disk;" before the call to nvme_nvm_register(),
>then, if register fails, and we jump to the put_disk(ns->disk) label,
>ns->disk will be NULL, so the recount will not be decreased, so I assume
>that this memory would then be a memory leak.
>
>
>I think that the problem is that the block functions are a bit messy.
>Most drivers seem to do blk_cleanup_queue() first and then do put_disk(),
>but some drivers do it in the opposite way, so I think that we might have
>some more use-after-free bugs in some of these drivers that do it in the
>opposite way.
>

Hi Niklas,

Yes, the out_put_disk label was introduced at the same time as the
LightNVM entry point. We can do a better job at separating the cleanup
functions, but as far as I can see ns->disk is not used in the LightNVM
initialization, so delaying the initialization should be ok. Part of
this should be also changing the out_put_disk to put_disk(disk).

Note that initializing other namespace types here do not require
ns->disk either, so delaying initialization should be ok. We have been
running with this patch locally for some time.

This said, this is an alternative as your fix works.

Javier