mm/rmap: clear vma->anon_vma on error

Commit 542eda1 ("mm/rmap: improve anon_vma_clone(),
unlink_anon_vmas() comments, add asserts") alters the way errors are
handled, but overlooked one important aspect of clean up.

When a VMA encounters an error state in anon_vma_clone() (that is, on
attempted allocation of anon_vma_chain objects), it cleans up partially
established state in cleanup_partial_anon_vmas(), before returning an
error.

However, this occurs prior to anon_vma->num_active_vmas being incremented,
and it also fails to clear the VMA's vma->anon_vma field, which remains in
place.

This is immediately an inconsistent state, because
anon_vma->num_active_vmas is supposed to track the number of VMAs whose
vma->anon_vma field references that anon_vma, and now that count is
off-by-negative-1 for each VMA for which this error state has occurred.

When VMAs are unlinked from this anon_vma, unlink_anon_vmas() will
eventually underflow anon_vma->num_active_vmas, which will trigger a
warning.

This will always eventually happen, as we unlink anon_vma's at process
teardown.

It could also cause maybe_reuse_anon_vma() to incorrectly permit the reuse
of an anon_vma which has active VMAs attached, which will lead to a
persistently invalid state.

The solution is to clear the VMA's anon_vma field when we clean up partial
state, as the fact we are doing so indicates clearly that the VMA is not
correctly integrated into the anon_vma tree and thus this field is
invalid.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 542eda1 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts")
Signed-off-by: Lorenzo Stoakes (Oracle) <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Closes: https://lore.kernel.org/linux-mm/[email protected]/
Reported-by: Jiakai Xu <[email protected]>
Closes: https://lore.kernel.org/linux-mm/CAFb8wJvRhatRD-9DVmr5v5pixTMPEr3UKjYBJjCd09OfH55CKg@mail.gmail.com/
Acked-by: David Hildenbrand (Arm) <[email protected]>
Acked-by: Vlastimil Babka (SUSE) <[email protected]>
Tested-by: Jiakai Xu <[email protected]>
Acked-by: Harry Yoo <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Sasha Levin (Microsoft) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Lorenzo Stoakes (Oracle)

mm/rmap.c

@@ -457,6 +457,13 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
+
+	/*
+	 * The anon_vma assigned to this VMA is no longer valid, as we were not
+	 * able to correctly clone AVC state. Avoid inconsistent anon_vma tree
+	 * state by resetting.
+	 */
+	vma->anon_vma = NULL;
 }
 
 /**