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

[Alex Williamson]

On Sat,  4 Jul 2026 18:40:10 -0700
Xiang Mei <xmei5@xxxxxxx> wrote:

> 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;
>  	}
>  

Applied to vfio next branch for v7.3.  Thanks,

Alex