sfc: fix use after free when disabling sriov

In the Linux kernel, the following vulnerability has been resolved:

sfc: fix use after free when disabling sriov

Use after free is detected by kfence when disabling sriov. What was read

after being freed was vf->pci_dev: it was freed from pci_disable_sriov

and later read in efx_ef10_sriov_free_vf_vports, called from

efx_ef10_sriov_free_vf_vswitching.

Set the pointer to NULL at release time to not trying to read it later.

Reproducer and dmesg log (note that kfence doesn't detect it every time):

$ echo 1 > /sys/class/net/enp65s0f0np0/device/sriov_numvfs

$ echo 0 > /sys/class/net/enp65s0f0np0/device/sriov_numvfs

BUG: KFENCE: use-after-free read in efx_ef10_sriov_free_vf_vswitching+0x82/0x170 [sfc]

Use-after-free read at 0x00000000ff3c1ba5 (in kfence-#224):

efx_ef10_sriov_free_vf_vswitching+0x82/0x170 [sfc]

efx_ef10_pci_sriov_disable+0x38/0x70 [sfc]

efx_pci_sriov_configure+0x24/0x40 [sfc]

sriov_numvfs_store+0xfe/0x140

kernfs_fop_write_iter+0x11c/0x1b0

new_sync_write+0x11f/0x1b0

vfs_write+0x1eb/0x280

ksys_write+0x5f/0xe0

do_syscall_64+0x5c/0x80

entry_SYSCALL_64_after_hwframe+0x44/0xae

kfence-#224: 0x00000000edb8ef95-0x00000000671f5ce1, size=2792, cache=kmalloc-4k

allocated by task 6771 on cpu 10 at 3137.860196s:

pci_alloc_dev+0x21/0x60

pci_iov_add_virtfn+0x2a2/0x320

sriov_enable+0x212/0x3e0

efx_ef10_sriov_configure+0x67/0x80 [sfc]

efx_pci_sriov_configure+0x24/0x40 [sfc]

sriov_numvfs_store+0xba/0x140

kernfs_fop_write_iter+0x11c/0x1b0

new_sync_write+0x11f/0x1b0

vfs_write+0x1eb/0x280

ksys_write+0x5f/0xe0

do_syscall_64+0x5c/0x80

entry_SYSCALL_64_after_hwframe+0x44/0xae

freed by task 6771 on cpu 12 at 3170.991309s:

device_release+0x34/0x90

kobject_cleanup+0x3a/0x130

pci_iov_remove_virtfn+0xd9/0x120

sriov_disable+0x30/0xe0

efx_ef10_pci_sriov_disable+0x57/0x70 [sfc]

efx_pci_sriov_configure+0x24/0x40 [sfc]

sriov_numvfs_store+0xfe/0x140

kernfs_fop_write_iter+0x11c/0x1b0

new_sync_write+0x11f/0x1b0

vfs_write+0x1eb/0x280

ksys_write+0x5f/0xe0

do_syscall_64+0x5c/0x80

entry_SYSCALL_64_after_hwframe+0x44/0xae