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.

Link: https://lkml.kernel.org/r/[e-mail]
Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]/
Fixes: 47b16d0462a4 ("mm: abort vma_modify() on merge out of memory failure")
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Suggested-by: Pedro Falcato <[e-mail]>
Suggested-by: Jann Horn <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/userfaultfd.c

···
 					     unsigned long end)
 {
 	struct vm_area_struct *ret;
+	bool give_up_on_oom = false;
+
+	/*
+	 * If we are modifying only and not splitting, just give up on the merge
+	 * if OOM prevents us from merging successfully.
+	 */
+	if (start == vma->vm_start && end == vma->vm_end)
+		give_up_on_oom = true;
 
 	/* Reset ptes for the whole vma range if wr-protected */
 	if (userfaultfd_wp(vma))
···
 
 	ret = vma_modify_flags_uffd(vmi, prev, vma, start, end,
 				    vma->vm_flags & ~__VM_UFFD_FLAGS,
-				    NULL_VM_UFFD_CTX);
+				    NULL_VM_UFFD_CTX, give_up_on_oom);
 
 	/*
 	 * In the vma_merge() successful mprotect-like case 8:
···
 		new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
 		vma = vma_modify_flags_uffd(&vmi, prev, vma, start, vma_end,
 					    new_flags,
-					    (struct vm_userfaultfd_ctx){ctx});
+					    (struct vm_userfaultfd_ctx){ctx},
+					    /* give_up_on_oom = */false);
 		if (IS_ERR(vma))
 			return PTR_ERR(vma);
 

mm/vma.c

···
 /*
  * Actually perform the VMA merge operation.
  *
+ * IMPORTANT: We guarantee that, should vmg->give_up_on_oom is set, to not
+ * modify any VMAs or cause inconsistent state should an OOM condition arise.
+ *
  * Returns 0 on success, or an error value on failure.
  */
 static int commit_merge(struct vma_merge_struct *vmg)
···
 
 	init_multi_vma_prep(&vp, vma, vmg);
 
+	/*
+	 * If vmg->give_up_on_oom is set, we're safe, because we don't actually
+	 * manipulate any VMAs until we succeed at preallocation.
+	 *
+	 * Past this point, we will not return an error.
+	 */
 	if (vma_iter_prealloc(vmg->vmi, vma))
 		return -ENOMEM;
 
···
 		if (anon_dup)
 			unlink_anon_vmas(anon_dup);
 
-		vmg->state = VMA_MERGE_ERROR_NOMEM;
+		/*
+		 * We've cleaned up any cloned anon_vma's, no VMAs have been
+		 * modified, no harm no foul if the user requests that we not
+		 * report this and just give up, leaving the VMAs unmerged.
+		 */
+		if (!vmg->give_up_on_oom)
+			vmg->state = VMA_MERGE_ERROR_NOMEM;
 		return NULL;
 	}
 
···
 abort:
 	vma_iter_set(vmg->vmi, start);
 	vma_iter_load(vmg->vmi);
-	vmg->state = VMA_MERGE_ERROR_NOMEM;
+
+	/*
+	 * This means we have failed to clone anon_vma's correctly, but no
+	 * actual changes to VMAs have occurred, so no harm no foul - if the
+	 * user doesn't want this reported and instead just wants to give up on
+	 * the merge, allow it.
+	 */
+	if (!vmg->give_up_on_oom)
+		vmg->state = VMA_MERGE_ERROR_NOMEM;
 	return NULL;
 }
 
···
 		/* This should already have been checked by this point. */
 		VM_WARN_ON_VMG(!can_merge_remove_vma(next), vmg);
 		vma_start_write(next);
+		/*
+		 * In this case we don't report OOM, so vmg->give_up_on_mm is
+		 * safe.
+		 */
 		ret = dup_anon_vma(middle, next, &anon_dup);
 		if (ret)
 			return ret;
···
 	return 0;
 
 nomem:
-	vmg->state = VMA_MERGE_ERROR_NOMEM;
 	if (anon_dup)
 		unlink_anon_vmas(anon_dup);
+	/*
+	 * If the user requests that we just give upon OOM, we are safe to do so
+	 * here, as commit merge provides this contract to us. Nothing has been
+	 * changed - no harm no foul, just don't report it.
+	 */
+	if (!vmg->give_up_on_oom)
+		vmg->state = VMA_MERGE_ERROR_NOMEM;
 	return -ENOMEM;
 }
 
···
 	if (vmg_nomem(vmg))
 		return ERR_PTR(-ENOMEM);
 
+	/*
+	 * Split can fail for reasons other than OOM, so if the user requests
+	 * this it's probably a mistake.
+	 */
+	VM_WARN_ON(vmg->give_up_on_oom &&
+		   (vma->vm_start != start || vma->vm_end != end));
+
 	/* Split any preceding portion of the VMA. */
 	if (vma->vm_start < start) {
 		int err = split_vma(vmg->vmi, vma, start, 1);
···
 		       struct vm_area_struct *vma,
 		       unsigned long start, unsigned long end,
 		       unsigned long new_flags,
-		       struct vm_userfaultfd_ctx new_ctx)
+		       struct vm_userfaultfd_ctx new_ctx,
+		       bool give_up_on_oom)
 {
 	VMG_VMA_STATE(vmg, vmi, prev, vma, start, end);
 
 	vmg.flags = new_flags;
 	vmg.uffd_ctx = new_ctx;
+	if (give_up_on_oom)
+		vmg.give_up_on_oom = true;
 
 	return vma_modify(&vmg);
 }

mm/vma.h

···
 	 */
 	bool just_expand :1;
 
+	/*
+	 * If a merge is possible, but an OOM error occurs, give up and don't
+	 * execute the merge, returning NULL.
+	 */
+	bool give_up_on_oom :1;
+
 	/* Internal flags set during merge process: */
 
 	/*
···
 		       struct vm_area_struct *vma,
 		       unsigned long start, unsigned long end,
 		       unsigned long new_flags,
-		       struct vm_userfaultfd_ctx new_ctx);
+		       struct vm_userfaultfd_ctx new_ctx,
+		       bool give_up_on_oom);
 
 __must_check struct vm_area_struct
 *vma_merge_new_range(struct vma_merge_struct *vmg);