Re: [PATCH 2/2] hugepages: Fix use after free bug in "quota"handling

From: Andrew Morton
Date: Wed Mar 07 2012 - 19:27:31 EST


On Wed, 7 Mar 2012 15:48:14 +1100
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:

> hugetlbfs_{get,put}_quota() are badly named. They don't interact with the
> general quota handling code, and they don't much resemble its behaviour.
> Rather than being about maintaining limits on on-disk block usage by
> particular users, they are instead about maintaining limits on in-memory
> page usage (including anonymous MAP_PRIVATE copied-on-write pages)
> associated with a particular hugetlbfs filesystem instance.
>
> Worse, they work by having callbacks to the hugetlbfs filesystem code from
> the low-level page handling code, in particular from free_huge_page().
> This is a layering violation of itself, but more importantly, if the kernel
> does a get_user_pages() on hugepages (which can happen from KVM amongst
> others), then the free_huge_page() can be delayed until after the
> associated inode has already been freed. If an unmount occurs at the
> wrong time, even the hugetlbfs superblock where the "quota" limits are
> stored may have been freed.
>
> Andrew Barry proposed a patch to fix this by having hugepages, instead of
> storing a pointer to their address_space and reaching the superblock from
> there, had the hugepages store pointers directly to the superblock, bumping
> the reference count as appropriate to avoid it being freed. Andrew Morton
> rejected that version, however, on the grounds that it made the existing
> layering violation worse.
>
> This is a reworked version of Andrew's patch, which removes the extra, and
> some of the existing, layering violation. It works by introducing the
> concept of a hugepage "subpool" at the lower hugepage mm layer - that is
> a finite logical pool of hugepages to allocate from. hugetlbfs now creates
> a subpool for each filesystem instance with a page limit set, and a pointer
> to the subpool gets added to each allocated hugepage, instead of the
> address_space pointer used now. The subpool has its own lifetime and is
> only freed once all pages in it _and_ all other references to it (i.e.
> superblocks) are gone.
>
> subpools are optional - a NULL subpool pointer is taken by the code to mean
> that no subpool limits are in effect.
>
> Previous discussion of this bug found in: "Fix refcounting in hugetlbfs
> quota handling.". See: https://lkml.org/lkml/2011/8/11/28 or
> http://marc.info/?l=linux-mm&m=126928970510627&w=1
>
> v2: Fixed a bug spotted by Hillf Danton, and removed the extra parameter to
> alloc_huge_page() - since it already takes the vma, it is not necessary.

Looks good - thanks for doing this.

Some comments, nothing serious:

>
> ...
>
> @@ -896,38 +910,12 @@ hugetlbfs_fill_super(struct super_block *sb, void *data, int silent)
> sb->s_root = root;
> return 0;
> out_free:
> + if (sbinfo->spool)
> + kfree(sbinfo->spool);

kfree(NULL) is OK, and omitting the NULL test is idiomatic.

> kfree(sbinfo);
> return -ENOMEM;
> }
>
>
> ...
>
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -14,6 +14,15 @@ struct user_struct;
> #include <linux/shm.h>
> #include <asm/tlbflush.h>
>
> +struct hugepage_subpool {
> + spinlock_t lock;
> + long count;
> + long max_hpages, used_hpages;
> +};

Could we get this struct docmented please? Why it exists, what it is
used for. Basically a copy-n-paste from the (nice) changelog.

Also please doucment the fields. I'm sitting here reading the code
trying to work out what these three fields semantically *mean*. Armed
with enough information to understand the code, I then get to read it
all again :(

I don't think any of these things can go negative, and a negative page
count is absurd. So perhaps they should have an unsigned type.

And maybe a 32-bit type? How much memory is 2^32 hugepages, and does
the present code even work correctly in that situation?

> +struct hugepage_subpool *hugepage_new_subpool(long nr_blocks);
> +void hugepage_put_subpool(struct hugepage_subpool *spool);
> +
> int PageHuge(struct page *page);
>
>
> ...
>
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -53,6 +53,84 @@ static unsigned long __initdata default_hstate_size;
> */
> static DEFINE_SPINLOCK(hugetlb_lock);
>
> +static inline void unlock_or_release_subpool(struct hugepage_subpool *spool)

Strange name. unlock_and_release() would be closer?

> +{
> + bool free = (spool->count == 0) && (spool->used_hpages == 0);

I wish I knew what those fields do.

> + spin_unlock(&spool->lock);
> +
> + /* If no pages are used, and no other handles to the subpool
> + * remain, free the subpool the subpool remain */

Comment is mangled.

Please use the normal multiline comment layout? There are examples in
this file.

> + if (free)
> + kfree(spool);
> +}
> +
> +struct hugepage_subpool *hugepage_new_subpool(long nr_blocks)

nr_blocks is logically an unsigned thing. And maybe 32-bit.

> +{
> + struct hugepage_subpool *spool;
> +
> + spool = kmalloc(sizeof(*spool), GFP_KERNEL);
> + if (!spool)
> + return NULL;
> +
> + spin_lock_init(&spool->lock);
> + spool->count = 1;
> + spool->max_hpages = nr_blocks;
> + spool->used_hpages = 0;
> +
> + return spool;
> +}

hm, I wonder why we didn't support a modular hugetlbfs driver. Perhaps
because of core hugetlb code which fiddles around in hugetlbfs. I
wonder if this patch fixed that?

> +void hugepage_put_subpool(struct hugepage_subpool *spool)
> +{
> + spin_lock(&spool->lock);
> + BUG_ON(!spool->count);
> + spool->count--;
> + unlock_or_release_subpool(spool);
> +}

ah-hah! ->count is in fact a refcount? Then why didn't we call it
"refcount" to save me all that head scratching? Here I was thinking it
must count hugepages.

> +static int hugepage_subpool_get_pages(struct hugepage_subpool *spool,
> + long delta)
> +{
> + int ret = 0;
> +
> + if (!spool)
> + return 0;
> +
> + spin_lock(&spool->lock);
> + if ((spool->used_hpages + delta) <= spool->max_hpages) {
> + spool->used_hpages += delta;
> + } else {
> + ret = -ENOMEM;
> + }

Can lose all the braces here.

> + spin_unlock(&spool->lock);
> +
> + return ret;
> +}

And some documeentation would be nice. It's strange to have a
foo_get_pages() which doesn't get any pages! And what does it mean
when spool==0? Why would anyone call this function without a subpool?

`delta' could be an unsigned int?

> +static void hugepage_subpool_put_pages(struct hugepage_subpool *spool,
> + long delta)
> +{
> + if (!spool)
> + return;
> +
> + spin_lock(&spool->lock);
> + spool->used_hpages -= delta;
> + /* If hugetlbfs_put_super couldn't free spool due to
> + * an outstanding quota reference, free it now. */

This is the sole remaining use of the term "quota" in hugetlb (yay).
Can we make this one go away as well?

> + unlock_or_release_subpool(spool);
> +}
> +
> +static inline struct hugepage_subpool *subpool_inode(struct inode *inode)
> +{
> + return HUGETLBFS_SB(inode->i_sb)->spool;
> +}
> +
> +static inline struct hugepage_subpool *subpool_vma(struct vm_area_struct *vma)
> +{
> + return subpool_inode(vma->vm_file->f_dentry->d_inode);
> +}
> +
> /*
> * Region tracking -- allows tracking of reservations and instantiated pages
> * across the pages in a mapping.
> @@ -533,9 +611,9 @@ static void free_huge_page(struct page *page)
> */
> struct hstate *h = page_hstate(page);
> int nid = page_to_nid(page);
> - struct address_space *mapping;
> + struct hugepage_subpool *spool =
> + (struct hugepage_subpool *)page_private(page);

Could do

struct hugepage_subpool *spool;

spool = (struct hugepage_subpool *)page_private(page);

and save the Ingo-upsetting 80-col trickery.

I'm shocked that we weren't already using the .private field of hugetlb
head pages.

> - mapping = (struct address_space *) page_private(page);
> set_page_private(page, 0);
> page->mapping = NULL;
> BUG_ON(page_count(page));
> @@ -551,8 +629,7 @@ static void free_huge_page(struct page *page)
> enqueue_huge_page(h, page);
> }
> spin_unlock(&hugetlb_lock);
> - if (mapping)
> - hugetlb_put_quota(mapping, 1);
> + hugepage_subpool_put_pages(spool, 1);
> }
>
> static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>
> ...
>
> @@ -2316,7 +2395,7 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
> */
> address = address & huge_page_mask(h);
> pgoff = vma_hugecache_offset(h, vma, address);
> - mapping = (struct address_space *)page_private(page);
> + mapping = vma->vm_file->f_dentry->d_inode->i_mapping;

Does

mapping = vma->vm_file->f_mapping;

work? Avoiding all this pointer hopping is why we added f_mapping,
actually.

> /*
> * Take the mapping lock for the duration of the table walk. As
>
> ...
>

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