[PATCH] vfio/pci: clear vdev->msi_perm after freeing it on init failure

[Xiang Mei]

vfio_msi_cap_len() lazily allocates the per-device MSI permission table:

	vdev->msi_perm = kmalloc_obj(struct perm_bits, GFP_KERNEL_ACCOUNT);
	if (!vdev->msi_perm)
		return -ENOMEM;

	ret = init_pci_cap_msi_perm(vdev->msi_perm, len, flags);
	if (ret) {
		kfree(vdev->msi_perm);
		return ret;		/* vdev->msi_perm left dangling */
	}

When init_pci_cap_msi_perm() -> alloc_perm_bits() fails with -ENOMEM, the
error path frees vdev->msi_perm but leaves the freed pointer stored in
it. vdev->msi_perm is not re-zeroed later because struct
vfio_pci_core_device is per-device and persists across open/close cycles,
and the vfio_config_init() error path returns without calling
vfio_config_free(). So the dangling pointer outlives the failed open.

That leads to two use-after-frees on the same device:

1. Reuse. The next vfio_config_init() sees the stale pointer at
   "if (vdev->msi_perm) return len;" and reuses the freed object. MSI
   config accesses in vfio_pci_config_rw_single() then dereference and
   call the freed perm->readfn / perm->writefn function pointers.

2. Double free. A later vfio_config_free() runs free_perm_bits() and
   kfree() on the already-freed object.

Fix it by NULLing vdev->msi_perm after the kfree(), matching the
NULL-after-free discipline already used in free_perm_bits() and
vfio_config_free().

  BUG: KASAN: slab-use-after-free in vfio_pci_config_rw_single (drivers/vfio/pci/vfio_pci_config.c:1961)
  Read of size 8 at addr ffff88800fcc88d0 by task exploit/143
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   vfio_pci_config_rw_single (drivers/vfio/pci/vfio_pci_config.c:1961)
   vfio_pci_config_rw (drivers/vfio/pci/vfio_pci_config.c:1986)
   vfio_pci_rw (drivers/vfio/pci/vfio_pci_core.c:1599)
   vfs_read (fs/read_write.c:572)
   __x64_sys_pread64 (fs/read_write.c:764)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   ...

Followed on device close by a double free of the same object:

  Oops: general protection fault, probably for non-canonical address
    0x1f63e0e8000008: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:kfree (mm/slub.c:6711)
  Call Trace:
   vfio_config_free (drivers/vfio/pci/vfio_pci_config.c:1861)
   vfio_pci_core_disable (drivers/vfio/pci/vfio_pci_core.c:685)
   vfio_pci_core_close_device (drivers/vfio/pci/vfio_pci_core.c:777)
   vfio_df_close (drivers/vfio/vfio_main.c:602)
   vfio_device_fops_release (drivers/vfio/vfio_main.c:648)
   __fput (fs/file_table.c:512)
   __x64_sys_close (fs/open.c:1496)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   ...
  Kernel panic - not syncing: Fatal exception

Fixes: 30ea32ab1951 ("vfio/pci: Fix potential memory leak in vfio_msi_cap_len")
Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
 drivers/vfio/pci/vfio_pci_config.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
index a10ed733f0e3..9914f3ac69ae 100644
--- a/drivers/vfio/pci/vfio_pci_config.c
+++ b/drivers/vfio/pci/vfio_pci_config.c
@@ -1272,6 +1272,7 @@ static int vfio_msi_cap_len(struct vfio_pci_core_device *vdev, u8 pos)
 	ret = init_pci_cap_msi_perm(vdev->msi_perm, len, flags);
 	if (ret) {
 		kfree(vdev->msi_perm);
+		vdev->msi_perm = NULL;
 		return ret;
 	}
 
-- 
2.43.0