Re: [PATCH] PCI/ASPM: fix UAF by disable ASPM for link when child function is removed

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On 2023/5/6 4:58, Bjorn Helgaas wrote:
On Thu, May 04, 2023 at 08:34:18PM +0800, Ding Hui wrote:
If the Function 0 of a Multi-Function device is software removed,
a freed downstream pointer will be left in struct pcie_link_state,
and then when pcie_config_aspm_link() be invoked from any path,
we will trigger use-after-free.

Based on the PCIe spec about ASPM Control (PCIe r6.0, sec,
for Multi-Function Devices (including ARI Devices), it is recommended
that software program the same value in all Functions. For ARI
Devices, ASPM Control is determined solely by the setting in Function 0.

So we can just disable ASPM of the whole component if any child
function is removed, the downstream pointer will be avoided from
use-after-free, that will also avoid other potential corner cases.

Fixes: b5a0a9b59c81 ("PCI/ASPM: Read and set up L1 substate capabilities")
Debugged-by: Zongquan Qin <qinzongquan@xxxxxxxxxxxxxx>
Suggestion-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
  drivers/pci/pcie/aspm.c | 15 +++++++--------
  1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 66d7514ca111..1bf8306141aa 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -1010,18 +1010,17 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)

Not directly related to your patch, but this looks racy to me:

   void pcie_aspm_exit_link_state(struct pci_dev *pdev)
     struct pci_dev *parent = pdev->bus->self;

     if (!parent || !parent->link_state)


     link = parent->link_state;
     root = link->root;
       link->pdev->link_state = NULL;

Since we check "parent->link_state" before acquiring the locks, I
suspect that removing two functions at the same time could end up with
a NULL pointer dereference:

   pcie_aspm_exit_link_state(fn 0)    pcie_aspm_exit_link_state(fn 1)
   parent = X                         parent = X
   parent->link_state != NULL         parent->link_state != NULL

   acquire locks
   link->pdev->link_state = NULL # aka parent->link_state
   release locks

                                      acquire locks
                                      link = parent->link_state # now NULL
                                      root = link->root         # NULL ptr

What do you think?  I guess if this *is* a race, it should be fixed by
a separate patch.

Maybe there's no need to worry about the race, when a pci device is removing,
the pci_rescan_remove_lock should be acquired.

Anyway, double check would be safer.

-	/*
-	 * All PCIe functions are in one slot, remove one function will remove
-	 * the whole slot, so just wait until we are the last function left.
-	 */
-	if (!list_empty(&parent->subordinate->devices))
-		goto out;
link = parent->link_state;
  	root = link->root;
  	parent_link = link->parent;
- /* All functions are removed, so just disable ASPM for the link */
+	/*
+	 * Any function is removed (including software removing), just
+	 * disable ASPM for the link, in case we can not configure the same
+	 * setting for all functions.
+	 * See PCIe r6.0, sec
+	 */
  	pcie_config_aspm_link(link, 0);
  	/* Clock PM is for endpoint device */
@@ -1032,7 +1031,7 @@ void pcie_aspm_exit_link_state(struct pci_dev *pdev)

- Ding Hui

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