If the vm_fault() handler raced with the vm_object_collapse()

sleepable scan, iteration over the shadow chain looking for a page
could find an OBJ_DEAD object.  Such state of the mapping is only
transient, the dead object will be terminated and removed from the
chain shortly.  We must not return KERN_PROTECTION_FAILURE unless the
object type is changed to OBJT_DEAD in the chain, indicating that
paging on this address is really impossible.  Returning
KERN_PROTECTION_FAILURE prematurely causes spurious SIGSEGV delivered
to processes, or kernel accesses to UVA spuriously failing with
EFAULT.

If the object with OBJ_DEAD flag is found, only return
KERN_PROTECTION_FAILURE when object type is already OBJT_DEAD.
Otherwise, sleep a tick and retry the fault handling.

Ideally, we would wait until the OBJ_DEAD flag is resolved, e.g. by
waiting until the paging on this object is finished.  But to do so, we
need to reference the dead object, while vm_object_collapse() insists
on owning the final reference on the collapsed object.  This could be
fixed by e.g. changing the assert to shared reference release between
vm_fault() and vm_object_collapse(), but it seems to be too much
complications for rare boundary condition.

PR:	204426
Tested by:    pho
Reviewed by:  alc
Sponsored by: The FreeBSD Foundation
X-Differential revision:	https://reviews.freebsd.org/D6085
MFC after:	2 weeks
Approved by:	re (gjb)

0 files changed, 0 insertions, 0 deletions