From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C85C51E3DC6 for ; Wed, 29 Jan 2025 21:29:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738186154; cv=none; b=q4oZm98my1S/iZBr6pZ9lp8oRmkyj4cbtJQCbQYf0n6reuNdQhPykOeY3ywDmQdc5/yoYXRkE/lFkkj57d2l1L75YZ/Q6ZSFQv5xQjHc9K4puRkzeW2VIyY9c4ywDcrHouEgsW+yRytg+ZmFmBsdHcci8NcBjsEfv2GRGw6GsNg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738186154; c=relaxed/simple; bh=P+h3+WMMqW1h+Os3rDcyrKh6SzSIMqvFhL3sJMTZ7PM=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition:In-Reply-To; b=V4gWmaGkL3/zS5uQqh3XLRt1vN5oUOszSUjUabHyqibMWHsvyiAlTNzOrvEo1YmyTfQYc25SGK1qAVM4wxpTavUe1CYDV7ZiR/YngaUIOMoRHU+jZc4RlDZhnoYC5m/HAP/xArJ+iG3Ne9OYFUVhOSolOjF/78Ydi+4P3F9J67A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bMN7eKQH; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="bMN7eKQH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1F885C4CED1; Wed, 29 Jan 2025 21:29:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738186154; bh=P+h3+WMMqW1h+Os3rDcyrKh6SzSIMqvFhL3sJMTZ7PM=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=bMN7eKQHsezazzvdmU1fXf4NDkIU2cPvpXHEOGCsV1yCug2H2kz02mC4H/hdlrcCV DgHVnBaF/+meWOXHj0AYMUYiwVuXxnlBuX5c8aQ4bWwQjikoCSJMyC9W7sThsnMpoQ lvNrXZkNKlUKeVYwmp6y02DVrPYz2oTdY3l8OCFv2xthTGkCCVtxfqjfKpLYI7iEyc 9W1EOEvqxKfsCKJ9YFYMrm/spqKkU7+I2klrE4UcViaZCsCR3jdMO4oOYMuck7RKmV 5e4TWv63FUbWfeGwNeE6mI2Pl8rzCsexZmjb0CgMLqo2yU5N+jycoNKUAdSR+wieTC /FB9ZvwkA7RHg== Date: Wed, 29 Jan 2025 15:29:12 -0600 From: Bjorn Helgaas To: Daniel Stodden Cc: dinghui@sangfor.com.cn, Daniel Stodden , bhelgaas@google.com, david.e.box@linux.intel.com, kai.heng.feng@canonical.com, linux-pci@vger.kernel.org, michael.a.bottini@linux.intel.com, qinzongquan@sangfor.com.cn, rajatja@google.com, refactormyself@gmail.com, sathyanarayanan.kuppuswamy@linux.intel.com, vidyas@nvidia.com Subject: Re: [PATCH 1/1] PCI/ASPM: fix link state exit during switch upstream function removal. Message-ID: <20250129212912.GA502577@bhelgaas> Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sun, Dec 22, 2024 at 07:39:08PM -0800, Daniel Stodden wrote: > From: Daniel Stodden > > Before change 456d8aa37d0f "Disable ASPM on MFD function removal to > avoid use-after-free", we would free the ASPM link only after the last > function on the bus pertaining to the given link was removed. > > That was too late. If function 0 is removed before sibling function, > link->downstream would point to free'd memory after. > > After above change, we freed the ASPM parent link state upon any > function removal on the bus pertaining to a given link. > > That is too early. If the link is to a PCIe switch with MFD on the > upstream port, then removing functions other than 0 first would free a > link which still remains parent_link to the remaining downstream > ports. Is this specific to a Switch? It seems like removal of any multi-function device might trip over this. > The resulting GPFs are especially frequent during hot-unplug, because > pciehp removes devices on the link bus in reverse order. Do you have a sample GPF? If we can include a few pertinent lines here it may help people connect a problem with this fix. > On that switch, function 0 is the virtual P2P bridge to the internal > bus. Free exactly when function 0 is removed -- before the parent link > is obsolete, but after all subordinate links are gone. I agree this is a problem. IIUC we allocate pcie_link_state when enumerating a device on the upstream end of a link, i.e., a Root Port or Switch Downstream Port, but we deallocate it when removing a device on the downstream end of the link, i.e., an Endpoint or Switch Upstream Port. This asymmetry seems kind of prone to error. Also, struct pcie_link_state contains redundant information, e.g., the pdev, downstream, parent, and sibling members basically duplicate the hierarchy already described by the struct pci_bus parent, self, and devices members. Redundancy like this is also error prone. This patch is attractive because it's a very small fix, and maybe we should use it for that reason. But I do think we're basically papering over a pretty serious design defect in ASPM. I think we'd ultimately be better off if we allocated pcie_link_state either as a member of struct pci_dev (instead of using a pointer), or perhaps in pci_setup_device() when we set up the rest of the bridge-related things and made it live as long as the bridge device. Actually, if we removed all the redundant pointers in struct pcie_link_state, it would be smaller than a single pointer, so there'd be no reason to allocate it dynamically. Of course this would be a much bigger change to aspm.c. > Fixes: 456d8aa37d0f ("PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free") Do we have any public problem reports we could mention here? I'm actually a little surprised that this hasn't been a bigger problem, given that 456d8aa37d0f appeared in v6.5 in Aug 2023. But maybe there is some unusual topology or hot-unplug involved? > Signed-off-by: Daniel Stodden > --- > drivers/pci/pcie/aspm.c | 17 +++++++++-------- > 1 file changed, 9 insertions(+), 8 deletions(-) > > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c > index e0bc90597dca..8ae7c75b408c 100644 > --- a/drivers/pci/pcie/aspm.c > +++ b/drivers/pci/pcie/aspm.c > @@ -1273,16 +1273,16 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev) > parent_link = link->parent; > > /* > - * link->downstream is a pointer to the pci_dev of function 0. If > - * we remove that function, the pci_dev is about to be deallocated, > - * so we can't use link->downstream again. Free the link state to > - * avoid this. > + * Free the parent link state, no later than function 0 (i.e. > + * link->downstream) being removed. > * > - * If we're removing a non-0 function, it's possible we could > - * retain the link state, but PCIe r6.0, sec 7.5.3.7, recommends > - * programming the same ASPM Control value for all functions of > - * multi-function devices, so disable ASPM for all of them. > + * Do not free free the link state any earlier. If function 0 > + * is a switch upstream port, this link state is parent_link > + * to all subordinate ones. > */ > + if (pdev != link->downstream) > + goto out; > + > pcie_config_aspm_link(link, 0); > list_del(&link->sibling); > free_link_state(link); > @@ -1293,6 +1293,7 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev) > pcie_config_aspm_path(parent_link); > } > > + out: > mutex_unlock(&aspm_lock); > up_read(&pci_bus_sem); > } > -- > 2.47.0 >