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[142.162.115.133]) by smtp.gmail.com with ESMTPSA id b16sm8170352qtx.85.2021.02.05.07.58.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Feb 2021 07:58:09 -0800 (PST) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1l83Um-003tDI-IH; Fri, 05 Feb 2021 11:58:08 -0400 Date: Fri, 5 Feb 2021 11:58:08 -0400 From: Jason Gunthorpe To: Lino Sanfilippo Cc: Lino Sanfilippo , peterhuewe@gmx.de, jarkko@kernel.org, stefanb@linux.vnet.ibm.com, James.Bottomley@hansenpartnership.com, stable@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip Message-ID: <20210205155808.GO4718@ziepe.ca> References: <1612482643-11796-1-git-send-email-LinoSanfilippo@gmx.de> <1612482643-11796-2-git-send-email-LinoSanfilippo@gmx.de> <20210205130511.GI4718@ziepe.ca> <3b821bf9-0f54-3473-d934-61c0c29f8957@kunbus.com> <20210205151511.GM4718@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-integrity@vger.kernel.org On Fri, Feb 05, 2021 at 04:50:13PM +0100, Lino Sanfilippo wrote: > > On 05.02.21 16:15, Jason Gunthorpe wrote: > > > > No, the cdev layer holds the refcount on the device while open is > > being called. > > > Yes, but the reference that is responsible for the chip deallocation is chip->dev > which is linked to chip->cdev and represents /dev/tpm, not /dev/tpmrm. > You are right, we dont have the issue with /dev/tpm for the reason you mentioned. > But /dev/tpmrm is represented by chip->cdevs and keeping this ref held by the cdev > layer wont protect us from the chip being freed (which is the reason why we need > the chip->dev reference in the first place). No, they are all chained together because they are all in the same struct: struct tpm_chip { struct device dev; struct device devs; struct cdev cdev; struct cdev cdevs; dev holds the refcount on memory, when it goes 0 the whole thing is kfreed. The rule is dev's refcount can't go to zero while any other refcount is != 0. For instance devs holds a get on dev that is put back only when devs goes to 0: static void tpm_devs_release(struct device *dev) { struct tpm_chip *chip = container_of(dev, struct tpm_chip, devs); /* release the master device reference */ put_device(&chip->dev); } Both cdev elements do something similar inside the cdev layer. The net result is during any open() the tpm_chip is guarenteed to have a positive refcount. Jason