It's possible the migration file is accessed after reset when it has
been cleaned up, especially when it's initiated by the device. This is
because the driver doesn't rip out the filep when cleaning up it only
frees the related page structures and sets its local struct
pds_vfio_lm_file pointer to NULL. This can cause a NULL pointer
dereference, which is shown in the example below during a restore after
a device initiated reset:

BUG: kernel NULL pointer dereference, address: 000000000000000c
PF: supervisor read access in kernel mode
PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
RIP: 0010:pds_vfio_get_file_page+0x5d/0xf0 [pds_vfio_pci]
[...]
Call Trace:
 <TASK>
 pds_vfio_restore_write+0xf6/0x160 [pds_vfio_pci]
 vfs_write+0xc9/0x3f0
 ? __fget_light+0xc9/0x110
 ksys_write+0xb5/0xf0
 __x64_sys_write+0x1a/0x20
 do_syscall_64+0x38/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]

Add a disabled flag to the driver's struct pds_vfio_lm_file that gets
set during cleanup. Then make sure to check the flag when the migration
file is accessed via its file_operations. By default this flag will be
false as the memory for struct pds_vfio_lm_file is kzalloc'd, which means
the struct pds_vfio_lm_file is enabled and accessible. Also, since the
file_operations and driver's migration file cleanup happen under the
protection of the same pds_vfio_lm_file.lock, using this flag is thread
safe.

Fixes: 8512ed25 ("vfio/pds: Always clear the save/restore FDs on reset")
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Link: https://lore.kernel.org/r/20240308182149.22036-2-brett.creeley@amd.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>