mm/vma: add give_up_on_oom option on modify/merge, use in uffd release

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

mm/vma: add give_up_on_oom option on modify/merge, use in uffd release

Currently, if a VMA merge fails due to an OOM condition arising on commit

merge or a failure to duplicate anon_vma's, we report this so the caller

can handle it.

However there are cases where the caller is only ostensibly trying a

merge, and doesn't mind if it fails due to this condition.

Since we do not want to introduce an implicit assumption that we only

actually modify VMAs after OOM conditions might arise, add a 'give up on

oom' option and make an explicit contract that, should this flag be set, we

absolutely will not modify any VMAs should OOM arise and just bail out.

Since it'd be very unusual for a user to try to vma_modify() with this flag

set but be specifying a range within a VMA which ends up being split (which

can fail due to rlimit issues, not only OOM), we add a debug warning for

this condition.

The motivating reason for this is uffd release - syzkaller (and Pedro

Falcato's VERY astute analysis) found a way in which an injected fault on

allocation, triggering an OOM condition on commit merge, would result in

uffd code becoming confused and treating an error value as if it were a VMA

pointer.

To avoid this, we make use of this new VMG flag to ensure that this never

occurs, utilising the fact that, should we be clearing entire VMAs, we do

not wish an OOM event to be reported to us.

Many thanks to Pedro Falcato for his excellent analysis and Jann Horn for

his insightful and intelligent analysis of the situation, both of whom were

instrumental in this fix.