Re: [PATCH] PCI/ASPM: Update ASPM sysfs on MFD function removal to avoid use-after-free

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Thu, 1 Aug 2024, Bjorn Helgaas wrote:

> On Thu, Aug 01, 2024 at 08:05:23PM +0800, Jay Fang wrote:
> > On 2024/8/1 5:46, Bjorn Helgaas wrote:
> > > On Tue, Jul 30, 2024 at 05:57:43PM +0800, Ding Hui wrote:
> > >> On 2024/7/30 9:16, Jay Fang wrote:
> > >>>  From 'commit 456d8aa37d0f ("PCI/ASPM: Disable ASPM on MFD function removal
> > >>> to avoid use-after-free")' we know that PCIe spec r6.0, sec 7.5.3.7,
> > >>> recommends that software program the same ASPM Control(pcie_link_state)
> > >>> value in all functions of multi-function devices, and free the
> > >>> pcie_link_state when any child function is removed.
> > >>>
> > >>> However, ASPM Control sysfs is still visible to other children even if it
> > >>> has been removed by any child function, and careless use it will
> > >>> trigger use-after-free error, e.g.:
> > >>>
> > >>>    # lspci -tv
> > >>>      -[0000:16]---00.0-[17]--+-00.0  Device 19e5:0222
> > >>>                              \-00.1  Device 19e5:0222
> > >>>    # echo 1 > /sys/bus/pci/devices/0000:17:00.0/remove       // pcie_link_state will be released
> > >>>    # echo 1 > /sys/bus/pci/devices/0000:17:00.1/link/l1_aspm // will trigger error
> > >>>
> > >>>    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030
> > >>>    Call trace:
> > >>>     aspm_attr_store_common.constprop.0+0x10c/0x154
> > >>>     l1_aspm_store+0x24/0x30
> > >>>     dev_attr_store+0x20/0x34
> > >>>     sysfs_kf_write+0x4c/0x5c
> > >>>
> > >>> We can solve this problem by updating the ASPM Control sysfs of all
> > >>> children immediately after ASPM Control have been freed.
> > >>>
> > >>> Fixes: 456d8aa37d0f ("PCI/ASPM: Disable ASPM on MFD function removal to avoid use-after-free")
> > >>> Signed-off-by: Jay Fang <f.fangjian@huawei.com>
> > >>> ---
> > >>>   drivers/pci/pcie/aspm.c | 2 ++
> > >>>   1 file changed, 2 insertions(+)
> > >>>
> > >>> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > >>> index cee2365e54b8..eee9e6739924 100644
> > >>> --- a/drivers/pci/pcie/aspm.c
> > >>> +++ b/drivers/pci/pcie/aspm.c
> > >>> @@ -1262,6 +1262,8 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)
> > >>>   		pcie_config_aspm_path(parent_link);
> > >>>   	}
> > >>> +	pcie_aspm_update_sysfs_visibility(parent);
> > >>> +
> > >>
> > >> To be more rigorous, is there still a race window in
> > >> aspm_attr_{show,store}_common or clkpm_{show,store} before updating
> > >> the visibility, we can get an old or NULL pointer by
> > >> pcie_aspm_get_link()?
> > > 
> > > Yeah, I think we still have a problem even with this patch.
> >
> > If so, maybe we need a new solution to completely sovle this problem.
> 
> I think so.  The pcie_link_state struct is kind of problematic to
> begin with.  It basically encodes the PCI hierarchy again, even though
> the hierarchy is already completely described via struct pci_dev.
> 
> IMO only the ASPM and clock PM state is really new information.  I'm
> not convinced that we even need all of that (how can
> supported/enabled/capable/default/disabled *all* be useful and
> understandable?).  But even if we *do* need all of that, it's only 39
> bits of information per device.

Hi all,

To me, the most natural place for the link-related information such as 
ASPM state would be inside struct pci_bus.

I actually did already take a look into migrating ASPM data there but the
way pcie_link_state is currently looked up through pci_dev (from both 
ends of the link) seemed to make the conversion somewhat messy so I 
postponed creating the patch for the migration.

But it's certainly a change I'd like to see if somebody wants to look into 
it.

-- 
 i.

> > > For one thing, aspm_attr_store_common() captures the pointer from
> > > pcie_aspm_get_link() before the critical section, so by the time it
> > > *uses* the pointer, pcie_aspm_exit_link_state() may have freed the
> > > link state.
> > > 
> > > And there are several other callers of pcie_aspm_get_link() that
> > > either call it before a critical section or don't have a critical
> > > section at all.
> > > 
> > > I think it may be a mistake to alloc/free the link state separately
> > > from the pci_dev itself.
> > > 
> > >>>   	mutex_unlock(&aspm_lock);
> > >>>   	up_read(&pci_bus_sem);
> > >>>   }
> > >>
> > >> -- 
> > >> Thanks,
> > >> - Ding Hui
> > >>
> > > 
> > > .
> > > 
> > 
>