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mm: rework vm_ops->close() handling on VMA merge
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In commit 714965c ("mm/mmap: start distinguishing if vma can be
removed in mergeability test") we relaxed the VMA merge rules for VMAs
possessing a vm_ops->close() hook, permitting this operation in instances
where we wouldn't delete the VMA as part of the merge operation.

This was later corrected in commit fc0c8f9 ("mm, mmap: fix
vma_merge() case 7 with vma_ops->close") to account for a subtle case that
the previous commit had not taken into account.

In both instances, we first rely on is_mergeable_vma() to determine
whether we might be dealing with a VMA that might be removed, taking
advantage of the fact that a 'previous' VMA will never be deleted, only
VMAs that follow it.

The second patch corrects the instance where a merge of the previous VMA
into a subsequent one did not correctly check whether the subsequent VMA
had a vm_ops->close() handler.

Both changes prevent merge cases that are actually permissible (for
instance a merge of a VMA into a following VMA with a vm_ops->close(), but
with no previous VMA, which would result in the next VMA being extended,
not deleted).

In addition, both changes fail to consider the case where a VMA that would
otherwise be merged with the previous and next VMA might have
vm_ops->close(), on the assumption that for this to be the case, all three
would have to have the same vma->vm_file to be mergeable and thus the same
vm_ops.

And in addition both changes operate at 50,000 feet, trying to guess
whether a VMA will be deleted.

As we have majorly refactored the VMA merge operation and de-duplicated
code to the point where we know precisely where deletions will occur, this
patch removes the aforementioned checks altogether and instead explicitly
checks whether a VMA will be deleted.

In cases where a reduced merge is still possible (where we merge both
previous and next VMA but the next VMA has a vm_ops->close hook, meaning
we could just merge the previous and current VMA), we do so, otherwise the
merge is not permitted.

We take advantage of our userland testing to assert that this functions
correctly - replacing the previous limited vm_ops->close() tests with
tests for every single case where we delete a VMA.

We also update all testing for both new and modified VMAs to set
vma->vm_ops->close() in every single instance where this would not prevent
the merge, to assert that we never do so.

Link: https://lkml.kernel.org/r/9f96b8cfeef3d14afabddac3d6144afdfbef2e22.1725040657.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Bert Karwatzki <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
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@lorenzo-stoakes @akpm00
lorenzo-stoakes authored and akpm00 committed Sep 4, 2024
1 parent cc8cb36 commit 01c373e
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Showing 2 changed files with 164 additions and 59 deletions.
57 changes: 36 additions & 21 deletions mm/vma.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -10,14 +10,6 @@
static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_next)
{
struct vm_area_struct *vma = merge_next ? vmg->next : vmg->prev;
/*
* If the vma has a ->close operation then the driver probably needs to
* release per-vma resources, so we don't attempt to merge those if the
* caller indicates the current vma may be removed as part of the merge,
* which is the case if we are attempting to merge the next VMA into
* this one.
*/
bool may_remove_vma = merge_next;

if (!mpol_equal(vmg->policy, vma_policy(vma)))
return false;
Expand All @@ -33,8 +25,6 @@ static inline bool is_mergeable_vma(struct vma_merge_struct *vmg, bool merge_nex
return false;
if (vma->vm_file != vmg->file)
return false;
if (may_remove_vma && vma->vm_ops && vma->vm_ops->close)
return false;
if (!is_mergeable_vm_userfaultfd_ctx(vma, vmg->uffd_ctx))
return false;
if (!anon_vma_name_eq(anon_vma_name(vma), vmg->anon_name))
Expand Down Expand Up @@ -632,6 +622,12 @@ static int commit_merge(struct vma_merge_struct *vmg,
return 0;
}

/* We can only remove VMAs when merging if they do not have a close hook. */
static bool can_merge_remove_vma(struct vm_area_struct *vma)
{
return !vma->vm_ops || !vma->vm_ops->close;
}

/*
* vma_merge_existing_range - Attempt to merge VMAs based on a VMA having its
* attributes modified.
Expand Down Expand Up @@ -725,12 +721,30 @@ static struct vm_area_struct *vma_merge_existing_range(struct vma_merge_struct *
merge_both = merge_left && merge_right;
/* If we span the entire VMA, a merge implies it will be deleted. */
merge_will_delete_vma = left_side && right_side;

/*
* If we need to remove vma in its entirety but are unable to do so,
* we have no sensible recourse but to abort the merge.
*/
if (merge_will_delete_vma && !can_merge_remove_vma(vma))
return NULL;

/*
* If we merge both VMAs, then next is also deleted. This implies
* merge_will_delete_vma also.
*/
merge_will_delete_next = merge_both;

/*
* If we cannot delete next, then we can reduce the operation to merging
* prev and vma (thereby deleting vma).
*/
if (merge_will_delete_next && !can_merge_remove_vma(next)) {
merge_will_delete_next = false;
merge_right = false;
merge_both = false;
}

/* No matter what happens, we will be adjusting vma. */
vma_start_write(vma);

Expand Down Expand Up @@ -772,21 +786,12 @@ static struct vm_area_struct *vma_merge_existing_range(struct vma_merge_struct *
vmg->start = prev->vm_start;
vmg->pgoff = prev->vm_pgoff;

if (merge_will_delete_vma) {
/*
* can_vma_merge_after() assumed we would not be
* removing vma, so it skipped the check for
* vm_ops->close, but we are removing vma.
*/
if (vma->vm_ops && vma->vm_ops->close)
err = -EINVAL;
} else {
if (!merge_will_delete_vma) {
adjust = vma;
adj_start = vmg->end - vma->vm_start;
}

if (!err)
err = dup_anon_vma(prev, vma, &anon_dup);
err = dup_anon_vma(prev, vma, &anon_dup);
} else { /* merge_right */
/*
* |<----->| OR
Expand Down Expand Up @@ -940,6 +945,14 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
vmg->vma = prev;
vmg->pgoff = prev->vm_pgoff;

/*
* If this merge would result in removal of the next VMA but we
* are not permitted to do so, reduce the operation to merging
* prev and vma.
*/
if (can_merge_right && !can_merge_remove_vma(next))
vmg->end = end;

vma_prev(vmg->vmi); /* Equivalent to going to the previous range */
}

Expand Down Expand Up @@ -994,6 +1007,8 @@ int vma_expand(struct vma_merge_struct *vmg)
int ret;

remove_next = true;
/* This should already have been checked by this point. */
VM_WARN_ON(!can_merge_remove_vma(next));
vma_start_write(next);
ret = dup_anon_vma(vma, next, &anon_dup);
if (ret)
Expand Down
166 changes: 128 additions & 38 deletions tools/testing/vma/vma.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -387,6 +387,9 @@ static bool test_merge_new(void)
struct anon_vma_chain dummy_anon_vma_chain_d = {
.anon_vma = &dummy_anon_vma,
};
const struct vm_operations_struct vm_ops = {
.close = dummy_close,
};
int count;
struct vm_area_struct *vma, *vma_a, *vma_b, *vma_c, *vma_d;
bool merged;
Expand Down Expand Up @@ -430,6 +433,7 @@ static bool test_merge_new(void)
* 0123456789abc
* AA*B DD CC
*/
vma_a->vm_ops = &vm_ops; /* This should have no impact. */
vma_b->anon_vma = &dummy_anon_vma;
vma = try_merge_new_vma(&mm, &vmg, 0x2000, 0x3000, 2, flags, &merged);
ASSERT_EQ(vma, vma_a);
Expand Down Expand Up @@ -466,6 +470,7 @@ static bool test_merge_new(void)
* AAAAA *DD CC
*/
vma_d->anon_vma = &dummy_anon_vma;
vma_d->vm_ops = &vm_ops; /* This should have no impact. */
vma = try_merge_new_vma(&mm, &vmg, 0x6000, 0x7000, 6, flags, &merged);
ASSERT_EQ(vma, vma_d);
/* Prepend. */
Expand All @@ -483,6 +488,7 @@ static bool test_merge_new(void)
* 0123456789abc
* AAAAA*DDD CC
*/
vma_d->vm_ops = NULL; /* This would otherwise degrade the merge. */
vma = try_merge_new_vma(&mm, &vmg, 0x5000, 0x6000, 5, flags, &merged);
ASSERT_EQ(vma, vma_a);
/* Merge with A, delete D. */
Expand Down Expand Up @@ -640,13 +646,11 @@ static bool test_vma_merge_with_close(void)
const struct vm_operations_struct vm_ops = {
.close = dummy_close,
};
struct vm_area_struct *vma_next =
alloc_and_link_vma(&mm, 0x2000, 0x3000, 2, flags);
struct vm_area_struct *vma;
struct vm_area_struct *vma_prev, *vma_next, *vma;

/*
* When we merge VMAs we sometimes have to delete others as part of the
* operation.
* When merging VMAs we are not permitted to remove any VMA that has a
* vm_ops->close() hook.
*
* Considering the two possible adjacent VMAs to which a VMA can be
* merged:
Expand Down Expand Up @@ -697,28 +701,52 @@ static bool test_vma_merge_with_close(void)
* would be set too, and thus scenario A would pick this up.
*/

ASSERT_NE(vma_next, NULL);

/*
* SCENARIO A
* The only case of a new VMA merge that results in a VMA being deleted
* is one where both the previous and next VMAs are merged - in this
* instance the next VMA is deleted, and the previous VMA is extended.
*
* 0123
* *N
* If we are unable to do so, we reduce the operation to simply
* extending the prev VMA and not merging next.
*
* 0123456789
* PPP**NNNN
* ->
* 0123456789
* PPPPPPNNN
*/

/* Make the next VMA have a close() callback. */
vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma_next = alloc_and_link_vma(&mm, 0x5000, 0x9000, 5, flags);
vma_next->vm_ops = &vm_ops;

/* Our proposed VMA has characteristics that would otherwise be merged. */
vmg_set_range(&vmg, 0x1000, 0x2000, 1, flags);
vmg_set_range(&vmg, 0x3000, 0x5000, 3, flags);
ASSERT_EQ(merge_new(&vmg), vma_prev);
ASSERT_EQ(vmg.state, VMA_MERGE_SUCCESS);
ASSERT_EQ(vma_prev->vm_start, 0);
ASSERT_EQ(vma_prev->vm_end, 0x5000);
ASSERT_EQ(vma_prev->vm_pgoff, 0);

/* The next VMA having a close() operator should cause the merge to fail.*/
ASSERT_EQ(merge_new(&vmg), NULL);
ASSERT_EQ(vmg.state, VMA_MERGE_NOMERGE);
ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);

/* Now create the VMA so we can merge via modified flags */
vmg_set_range(&vmg, 0x1000, 0x2000, 1, flags);
vma = alloc_and_link_vma(&mm, 0x1000, 0x2000, 1, flags);
/*
* When modifying an existing VMA there are further cases where we
* delete VMAs.
*
* <>
* 0123456789
* PPPVV
*
* In this instance, if vma has a close hook, the merge simply cannot
* proceed.
*/

vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma = alloc_and_link_vma(&mm, 0x3000, 0x5000, 3, flags);
vma->vm_ops = &vm_ops;

vmg_set_range(&vmg, 0x3000, 0x5000, 3, flags);
vmg.prev = vma_prev;
vmg.vma = vma;

/*
Expand All @@ -728,38 +756,90 @@ static bool test_vma_merge_with_close(void)
ASSERT_EQ(merge_existing(&vmg), NULL);
ASSERT_EQ(vmg.state, VMA_MERGE_NOMERGE);

/* SCENARIO B
ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);

/*
* This case is mirrored if merging with next.
*
* 0123
* P*
* <>
* 0123456789
* VVNNNN
*
* In order for this scenario to trigger, the VMA currently being
* modified must also have a .close().
* In this instance, if vma has a close hook, the merge simply cannot
* proceed.
*/

/* Reset VMG state. */
vmg_set_range(&vmg, 0x1000, 0x2000, 1, flags);
/*
* Make next unmergeable, and don't let the scenario A check pick this
* up, we want to reproduce scenario B only.
*/
vma_next->vm_ops = NULL;
vma_next->__vm_flags &= ~VM_MAYWRITE;
/* Allocate prev. */
vmg.prev = alloc_and_link_vma(&mm, 0, 0x1000, 0, flags);
/* Assign a vm_ops->close() function to VMA explicitly. */
vma = alloc_and_link_vma(&mm, 0x3000, 0x5000, 3, flags);
vma_next = alloc_and_link_vma(&mm, 0x5000, 0x9000, 5, flags);
vma->vm_ops = &vm_ops;

vmg_set_range(&vmg, 0x3000, 0x5000, 3, flags);
vmg.vma = vma;
/* Make sure merge does not occur. */
ASSERT_EQ(merge_existing(&vmg), NULL);
/*
* Initially this is misapprehended as an out of memory report, as the
* close() check is handled in the same way as anon_vma duplication
* failures, however a subsequent patch resolves this.
*/
ASSERT_EQ(vmg.state, VMA_MERGE_ERROR_NOMEM);
ASSERT_EQ(vmg.state, VMA_MERGE_NOMERGE);

ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);

/*
* Finally, we consider two variants of the case where we modify a VMA
* to merge with both the previous and next VMAs.
*
* The first variant is where vma has a close hook. In this instance, no
* merge can proceed.
*
* <>
* 0123456789
* PPPVVNNNN
*/

vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma = alloc_and_link_vma(&mm, 0x3000, 0x5000, 3, flags);
vma_next = alloc_and_link_vma(&mm, 0x5000, 0x9000, 5, flags);
vma->vm_ops = &vm_ops;

vmg_set_range(&vmg, 0x3000, 0x5000, 3, flags);
vmg.prev = vma_prev;
vmg.vma = vma;

ASSERT_EQ(merge_existing(&vmg), NULL);
ASSERT_EQ(vmg.state, VMA_MERGE_NOMERGE);

ASSERT_EQ(cleanup_mm(&mm, &vmi), 3);

/*
* The second variant is where next has a close hook. In this instance,
* we reduce the operation to a merge between prev and vma.
*
* <>
* 0123456789
* PPPVVNNNN
* ->
* 0123456789
* PPPPPNNNN
*/

vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma = alloc_and_link_vma(&mm, 0x3000, 0x5000, 3, flags);
vma_next = alloc_and_link_vma(&mm, 0x5000, 0x9000, 5, flags);
vma_next->vm_ops = &vm_ops;

vmg_set_range(&vmg, 0x3000, 0x5000, 3, flags);
vmg.prev = vma_prev;
vmg.vma = vma;

ASSERT_EQ(merge_existing(&vmg), vma_prev);
ASSERT_EQ(vmg.state, VMA_MERGE_SUCCESS);
ASSERT_EQ(vma_prev->vm_start, 0);
ASSERT_EQ(vma_prev->vm_end, 0x5000);
ASSERT_EQ(vma_prev->vm_pgoff, 0);

ASSERT_EQ(cleanup_mm(&mm, &vmi), 2);

cleanup_mm(&mm, &vmi);
return true;
}

Expand Down Expand Up @@ -828,6 +908,9 @@ static bool test_merge_existing(void)
.mm = &mm,
.vmi = &vmi,
};
const struct vm_operations_struct vm_ops = {
.close = dummy_close,
};

/*
* Merge right case - partial span.
Expand All @@ -840,7 +923,9 @@ static bool test_merge_existing(void)
* VNNNNNN
*/
vma = alloc_and_link_vma(&mm, 0x2000, 0x6000, 2, flags);
vma->vm_ops = &vm_ops; /* This should have no impact. */
vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, flags);
vma_next->vm_ops = &vm_ops; /* This should have no impact. */
vmg_set_range(&vmg, 0x3000, 0x6000, 3, flags);
vmg.vma = vma;
vmg.prev = vma;
Expand Down Expand Up @@ -873,6 +958,7 @@ static bool test_merge_existing(void)
*/
vma = alloc_and_link_vma(&mm, 0x2000, 0x6000, 2, flags);
vma_next = alloc_and_link_vma(&mm, 0x6000, 0x9000, 6, flags);
vma_next->vm_ops = &vm_ops; /* This should have no impact. */
vmg_set_range(&vmg, 0x2000, 0x6000, 2, flags);
vmg.vma = vma;
vma->anon_vma = &dummy_anon_vma;
Expand All @@ -899,7 +985,9 @@ static bool test_merge_existing(void)
* PPPPPPV
*/
vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, flags);
vma->vm_ops = &vm_ops; /* This should have no impact. */
vmg_set_range(&vmg, 0x3000, 0x6000, 3, flags);
vmg.prev = vma_prev;
vmg.vma = vma;
Expand Down Expand Up @@ -932,6 +1020,7 @@ static bool test_merge_existing(void)
* PPPPPPP
*/
vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, flags);
vmg_set_range(&vmg, 0x3000, 0x7000, 3, flags);
vmg.prev = vma_prev;
Expand Down Expand Up @@ -960,6 +1049,7 @@ static bool test_merge_existing(void)
* PPPPPPPPPP
*/
vma_prev = alloc_and_link_vma(&mm, 0, 0x3000, 0, flags);
vma_prev->vm_ops = &vm_ops; /* This should have no impact. */
vma = alloc_and_link_vma(&mm, 0x3000, 0x7000, 3, flags);
vma_next = alloc_and_link_vma(&mm, 0x7000, 0x9000, 7, flags);
vmg_set_range(&vmg, 0x3000, 0x7000, 3, flags);
Expand Down

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