[PATCH RFC 1/2] mm, madvise: simplify anon_name handling

[Vlastimil Babka <vbabka@suse.cz>]

Since the introduction in 9a10064f5625 ("mm: add a field to store names
for private anonymous memory") the code to set anon_name on a vma has
been using madvise_update_vma() to call replace_vma_anon_name(). Since
the former is called also by a number of other madvise behaviours that
do not set a new anon_name, they have been passing the existing
anon_name of the vma to make replace_vma_anon_name() a no-op.

This is rather wasteful as it needs anon_vma_name_eq() to determine the
no-op situations, and checks for when replace_vma_anon_name() is allowed
(the vma is anon/shmem) duplicate the checks already done earlier in
madvise_vma_behavior(). It has also lead to commit 942341dcc574 ("mm:
fix use-after-free when anon vma name is used after vma is freed")
adding anon_name refcount get/put operations exactly to the cases that
actually do not change anon_name - just so the replace_vma_anon_name()
can keep safely determining it has nothing to do.

The recent madvise cleanups made this suboptimal handling very obvious,
but happily also allow for an easy fix. madvise_update_vma() now has the
complete information whether it's been called to set a new anon_name, so
stop passing it the existing vma's name and doing the refcount get/put
in its only caller madvise_vma_behavior().

In madvise_update_vma() itself, limit calling of replace_anon_vma_name()
only to cases where we are setting a new name, otherwise we know it's a
no-op. We can rely solely on the __MADV_SET_ANON_VMA_NAME behaviour and
can remove the duplicate checks for vma being anon/shmem that were done
already in madvise_vma_behavior().

The remaining reason to obtain the vma's existing anon_name is to pass
it to vma_modify_flags_name() for the splitting and merging to work
properly. In case of merging, the vma might be freed along with the
anon_name, but madvise_update_vma() will not access it afterwards so the
UAF previously fixed by commit 942341dcc574 is not reintroduced.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/madvise.c | 37 +++++++++++++------------------------
 1 file changed, 13 insertions(+), 24 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 4491bf080f55d6d1aeffb2ff0b8fdd28904af950..ae29395b4fc7f65a449c5772b1901a90f4195885 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -176,21 +176,25 @@ static int replace_anon_vma_name(struct vm_area_struct *vma,
 }
 #endif /* CONFIG_ANON_VMA_NAME */
 /*
- * Update the vm_flags on region of a vma, splitting it or merging it as
- * necessary.  Must be called with mmap_lock held for writing;
- * Caller should ensure anon_name stability by raising its refcount even when
- * anon_name belongs to a valid vma because this function might free that vma.
+ * Update the vm_flags and/or anon_name on region of a vma, splitting it or
+ * merging it as necessary. Must be called with mmap_lock held for writing.
  */
 static int madvise_update_vma(vm_flags_t new_flags,
 		struct madvise_behavior *madv_behavior)
 {
-	int error;
 	struct vm_area_struct *vma = madv_behavior->vma;
 	struct madvise_behavior_range *range = &madv_behavior->range;
-	struct anon_vma_name *anon_name = madv_behavior->anon_name;
+	bool set_new_anon_name = madv_behavior->behavior == __MADV_SET_ANON_VMA_NAME;
+	struct anon_vma_name *anon_name;
 	VMA_ITERATOR(vmi, madv_behavior->mm, range->start);
 
-	if (new_flags == vma->vm_flags && anon_vma_name_eq(anon_vma_name(vma), anon_name))
+	if (set_new_anon_name)
+		anon_name = madv_behavior->anon_name;
+	else
+		anon_name = anon_vma_name(vma);
+
+	if (new_flags == vma->vm_flags && (!set_new_anon_name
+			|| anon_vma_name_eq(anon_vma_name(vma), anon_name)))
 		return 0;
 
 	vma = vma_modify_flags_name(&vmi, madv_behavior->prev, vma,
@@ -203,11 +207,8 @@ static int madvise_update_vma(vm_flags_t new_flags,
 	/* vm_flags is protected by the mmap_lock held in write mode. */
 	vma_start_write(vma);
 	vm_flags_reset(vma, new_flags);
-	if (!vma->vm_file || vma_is_anon_shmem(vma)) {
-		error = replace_anon_vma_name(vma, anon_name);
-		if (error)
-			return error;
-	}
+	if (set_new_anon_name)
+		return replace_anon_vma_name(vma, anon_name);
 
 	return 0;
 }
@@ -1313,7 +1314,6 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
 	int behavior = madv_behavior->behavior;
 	struct vm_area_struct *vma = madv_behavior->vma;
 	vm_flags_t new_flags = vma->vm_flags;
-	bool set_new_anon_name = behavior == __MADV_SET_ANON_VMA_NAME;
 	struct madvise_behavior_range *range = &madv_behavior->range;
 	int error;
 
@@ -1403,18 +1403,7 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
 	/* This is a write operation.*/
 	VM_WARN_ON_ONCE(madv_behavior->lock_mode != MADVISE_MMAP_WRITE_LOCK);
 
-	/*
-	 * madvise_update_vma() might cause a VMA merge which could put an
-	 * anon_vma_name, so we must hold an additional reference on the
-	 * anon_vma_name so it doesn't disappear from under us.
-	 */
-	if (!set_new_anon_name) {
-		madv_behavior->anon_name = anon_vma_name(vma);
-		anon_vma_name_get(madv_behavior->anon_name);
-	}
 	error = madvise_update_vma(new_flags, madv_behavior);
-	if (!set_new_anon_name)
-		anon_vma_name_put(madv_behavior->anon_name);
 out:
 	/*
 	 * madvise() returns EAGAIN if kernel resources, such as

-- 
2.50.0