Message-ID: <878vjbaf3y.fsf@linux.vnet.ibm.com>
Date:	Thu, 08 Mar 2012 10:00:57 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	David Gibson <david@...son.dropbear.id.au>,
	akpm@...ux-foundation.org, hughd@...gle.com
Cc:	paulus@...ba.org, linux-kernel@...r.kernel.org,
	David Gibson <david@...son.dropbear.id.au>,
	Andrew Barry <abarry@...y.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH 2/2] hugepages: Fix use after free bug in "quota" handling

On Wed,  7 Mar 2012 15:48:14 +1100, David Gibson <david@...son.dropbear.id.au> 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.
> 
> Signed-off-by: Andrew Barry <abarry@...y.com>
> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Minchan Kim <minchan.kim@...il.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Hillf Danton <dhillf@...il.com>
> ---
>  fs/hugetlbfs/inode.c    |   54 +++++++-----------
>  include/linux/hugetlb.h |   14 ++++--
>  mm/hugetlb.c            |  135 +++++++++++++++++++++++++++++++++++++---------
>  3 files changed, 139 insertions(+), 64 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index bb0e366..74c6ba2 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -626,9 +626,15 @@ static int hugetlbfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  		spin_lock(&sbinfo->stat_lock);
>  		/* If no limits set, just report 0 for max/free/used
>  		 * blocks, like simple_statfs() */
> -		if (sbinfo->max_blocks >= 0) {
> -			buf->f_blocks = sbinfo->max_blocks;
> -			buf->f_bavail = buf->f_bfree = sbinfo->free_blocks;
> +		if (sbinfo->spool) {
> +			long free_pages;
> +
> +			spin_lock(&sbinfo->spool->lock);
> +			buf->f_blocks = sbinfo->spool->max_hpages;
> +			free_pages = sbinfo->spool->max_hpages
> +				- sbinfo->spool->used_hpages;
> +			buf->f_bavail = buf->f_bfree = free_pages;
> +			spin_unlock(&sbinfo->spool->lock);
>  			buf->f_files = sbinfo->max_inodes;
>  			buf->f_ffree = sbinfo->free_inodes;
>  		}
> @@ -644,6 +650,10 @@ static void hugetlbfs_put_super(struct super_block *sb)
> 
>  	if (sbi) {
>  		sb->s_fs_info = NULL;
> +
> +		if (sbi->spool)
> +			hugepage_put_subpool(sbi->spool);
> +
>  		kfree(sbi);
>  	}
>  }
> @@ -874,10 +884,14 @@ hugetlbfs_fill_super(struct super_block *sb, void *data, int silent)
>  	sb->s_fs_info = sbinfo;
>  	sbinfo->hstate = config.hstate;
>  	spin_lock_init(&sbinfo->stat_lock);
> -	sbinfo->max_blocks = config.nr_blocks;
> -	sbinfo->free_blocks = config.nr_blocks;
>  	sbinfo->max_inodes = config.nr_inodes;
>  	sbinfo->free_inodes = config.nr_inodes;
> +	sbinfo->spool = NULL;
> +	if (config.nr_blocks != -1) {
> +		sbinfo->spool = hugepage_new_subpool(config.nr_blocks);
> +		if (!sbinfo->spool)
> +			goto out_free;
> +	}
>  	sb->s_maxbytes = MAX_LFS_FILESIZE;
>  	sb->s_blocksize = huge_page_size(config.hstate);
>  	sb->s_blocksize_bits = huge_page_shift(config.hstate);
> @@ -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() should handle NULL 


>  	kfree(sbinfo);
>  	return -ENOMEM;
>  }
> 
> -int hugetlb_get_quota(struct address_space *mapping, long delta)
> -{
> -	int ret = 0;
> -	struct hugetlbfs_sb_info *sbinfo = HUGETLBFS_SB(mapping->host->i_sb);
> -
> -	if (sbinfo->free_blocks > -1) {
> -		spin_lock(&sbinfo->stat_lock);
> -		if (sbinfo->free_blocks - delta >= 0)
> -			sbinfo->free_blocks -= delta;
> -		else
> -			ret = -ENOMEM;
> -		spin_unlock(&sbinfo->stat_lock);
> -	}
> -
> -	return ret;
> -}
> -
> -void hugetlb_put_quota(struct address_space *mapping, long delta)
> -{
> -	struct hugetlbfs_sb_info *sbinfo = HUGETLBFS_SB(mapping->host->i_sb);
> -
> -	if (sbinfo->free_blocks > -1) {
> -		spin_lock(&sbinfo->stat_lock);
> -		sbinfo->free_blocks += delta;
> -		spin_unlock(&sbinfo->stat_lock);
> -	}
> -}
> -
>  static struct dentry *hugetlbfs_mount(struct file_system_type *fs_type,
>  	int flags, const char *dev_name, void *data)
>  {
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 7adc492..cf01817 100644
> --- 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;
> +};
> +
> +struct hugepage_subpool *hugepage_new_subpool(long nr_blocks);
> +void hugepage_put_subpool(struct hugepage_subpool *spool);
> +
>  int PageHuge(struct page *page);
> 
>  void reset_vma_resv_huge_pages(struct vm_area_struct *vma);
> @@ -129,12 +138,11 @@ enum {
> 
>  #ifdef CONFIG_HUGETLBFS
>  struct hugetlbfs_sb_info {
> -	long	max_blocks;   /* blocks allowed */
> -	long	free_blocks;  /* blocks free */
>  	long	max_inodes;   /* inodes allowed */
>  	long	free_inodes;  /* inodes free */
>  	spinlock_t	stat_lock;
>  	struct hstate *hstate;
> +	struct hugepage_subpool *spool;
>  };
> 
>  static inline struct hugetlbfs_sb_info *HUGETLBFS_SB(struct super_block *sb)
> @@ -146,8 +154,6 @@ extern const struct file_operations hugetlbfs_file_operations;
>  extern const struct vm_operations_struct hugetlb_vm_ops;
>  struct file *hugetlb_file_setup(const char *name, size_t size, vm_flags_t acct,
>  				struct user_struct **user, int creat_flags);
> -int hugetlb_get_quota(struct address_space *mapping, long delta);
> -void hugetlb_put_quota(struct address_space *mapping, long delta);
> 
>  static inline int is_file_hugepages(struct file *file)
>  {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5f34bd8..36b38b3a 100644
> --- 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)
> +{
> +	bool free = (spool->count == 0) && (spool->used_hpages == 0);
> +

I have not see that style in other part of kernel. May be with proper

if (spool->count == 0 && spool->used_hpages == 0)
   free = 1

> +	spin_unlock(&spool->lock);
> +


Having the spin_lock held across functions is also strange. Since there
are only two callers, may be this can be inlined in the callers ?


> +	/* If no pages are used, and no other handles to the subpool
> +	 * remain, free the subpool the subpool remain */
> +	if (free)
> +		kfree(spool);
> +}
> +
> +struct hugepage_subpool *hugepage_new_subpool(long nr_blocks)
> +{
> +	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;
> +}
> +
> +void hugepage_put_subpool(struct hugepage_subpool *spool)
> +{
> +	spin_lock(&spool->lock);
> +	BUG_ON(!spool->count);
> +	spool->count--;
> +	unlock_or_release_subpool(spool);
> +}
> +
> +static int hugepage_subpool_get_pages(struct hugepage_subpool *spool,
> +				      long delta)

s/long delta/long hpage/ or something like that ?


> +{
> +	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;
> +	}
> +	spin_unlock(&spool->lock);
> +
> +	return ret;
> +}
> +
> +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. */
> +	unlock_or_release_subpool(spool);
> +}
> +
> +static inline struct hugepage_subpool *subpool_inode(struct inode
> *inode)

s/subpool_inode/subpool_from_inode/ ?

> +{
> +	return HUGETLBFS_SB(inode->i_sb)->spool;
> +}
> +
> +static inline struct hugepage_subpool *subpool_vma(struct
> vm_area_struct *vma)

s/subpool_vma/subpool_from_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);
> 
> -	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);


We would still need the if () checking there. When we do echo x >
/proc/sys/vm/nr_hugepages we would call free_huge_page to prepare new
huge page pool. But we don't have page_private set for them.


>  }
> 
>  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> @@ -966,11 +1043,12 @@ static void return_unused_surplus_pages(struct hstate *h,
>  /*
>   * Determine if the huge page at addr within the vma has an associated
>   * reservation.  Where it does not we will need to logically increase
> - * reservation and actually increase quota before an allocation can occur.
> - * Where any new reservation would be required the reservation change is
> - * prepared, but not committed.  Once the page has been quota'd allocated
> - * an instantiated the change should be committed via vma_commit_reservation.
> - * No action is required on failure.
> + * reservation and actually increase subpool usage before an allocation
> + * can occur.  Where any new reservation would be required the
> + * reservation change is prepared, but not committed.  Once the page
> + * has been allocated from the subpool and instantiated the change should
> + * be committed via vma_commit_reservation.  No action is required on
> + * failure.
>   */
>  static long vma_needs_reservation(struct hstate *h,
>  			struct vm_area_struct *vma, unsigned long addr)
> @@ -1019,24 +1097,24 @@ static void vma_commit_reservation(struct hstate *h,
>  static struct page *alloc_huge_page(struct vm_area_struct *vma,
>  				    unsigned long addr, int avoid_reserve)
>  {
> +	struct hugepage_subpool *spool = subpool_vma(vma);
>  	struct hstate *h = hstate_vma(vma);
>  	struct page *page;
> -	struct address_space *mapping = vma->vm_file->f_mapping;
> -	struct inode *inode = mapping->host;
>  	long chg;
> 
>  	/*
> -	 * Processes that did not create the mapping will have no reserves and
> -	 * will not have accounted against quota. Check that the quota can be
> -	 * made before satisfying the allocation
> -	 * MAP_NORESERVE mappings may also need pages and quota allocated
> -	 * if no reserve mapping overlaps.
> +	 * Processes that did not create the mapping will have no
> +	 * reserves and will not have accounted against subpool
> +	 * limit. Check that the subpool limit can be made before
> +	 * satisfying the allocation MAP_NORESERVE mappings may also
> +	 * need pages and subpool limit allocated allocated if no reserve
> +	 * mapping overlaps.
>  	 */
>  	chg = vma_needs_reservation(h, vma, addr);
>  	if (chg < 0)
>  		return ERR_PTR(-VM_FAULT_OOM);
>  	if (chg)
> -		if (hugetlb_get_quota(inode->i_mapping, chg))
> +		if (hugepage_subpool_get_pages(spool, chg))
>  			return ERR_PTR(-VM_FAULT_SIGBUS);
> 
>  	spin_lock(&hugetlb_lock);
> @@ -1046,12 +1124,12 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
>  	if (!page) {
>  		page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
>  		if (!page) {
> -			hugetlb_put_quota(inode->i_mapping, chg);
> +			hugepage_subpool_put_pages(spool, chg);
>  			return ERR_PTR(-VM_FAULT_SIGBUS);
>  		}
>  	}
> 
> -	set_page_private(page, (unsigned long) mapping);
> +	set_page_private(page, (unsigned long)spool);
> 
>  	vma_commit_reservation(h, vma, addr);
> 
> @@ -2072,6 +2150,7 @@ static void hugetlb_vm_op_close(struct vm_area_struct *vma)
>  {
>  	struct hstate *h = hstate_vma(vma);
>  	struct resv_map *reservations = vma_resv_map(vma);
> +	struct hugepage_subpool *spool = subpool_vma(vma);
>  	unsigned long reserve;
>  	unsigned long start;
>  	unsigned long end;
> @@ -2087,7 +2166,7 @@ static void hugetlb_vm_op_close(struct vm_area_struct *vma)
> 
>  		if (reserve) {
>  			hugetlb_acct_memory(h, -reserve);
> -			hugetlb_put_quota(vma->vm_file->f_mapping, reserve);
> +			hugepage_subpool_put_pages(spool, reserve);
>  		}
>  	}
>  }
> @@ -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;
> 
>  	/*
>  	 * Take the mapping lock for the duration of the table walk. As
> @@ -2869,11 +2948,12 @@ int hugetlb_reserve_pages(struct inode *inode,
>  {
>  	long ret, chg;
>  	struct hstate *h = hstate_inode(inode);
> +	struct hugepage_subpool *spool = subpool_inode(inode);
> 
>  	/*
>  	 * Only apply hugepage reservation if asked. At fault time, an
>  	 * attempt will be made for VM_NORESERVE to allocate a page
> -	 * and filesystem quota without using reserves
> +	 * without using reserves
>  	 */
>  	if (vm_flags & VM_NORESERVE)
>  		return 0;
> @@ -2900,17 +2980,17 @@ int hugetlb_reserve_pages(struct inode *inode,
>  	if (chg < 0)
>  		return chg;
> 
> -	/* There must be enough filesystem quota for the mapping */
> -	if (hugetlb_get_quota(inode->i_mapping, chg))
> +	/* There must be enough pages in the subpool for the mapping */
> +	if (hugepage_subpool_get_pages(spool, chg))
>  		return -ENOSPC;
> 
>  	/*
>  	 * Check enough hugepages are available for the reservation.
> -	 * Hand back the quota if there are not
> +	 * Hand the pages back to the subpool if there are not
>  	 */
>  	ret = hugetlb_acct_memory(h, chg);
>  	if (ret < 0) {
> -		hugetlb_put_quota(inode->i_mapping, chg);
> +		hugepage_subpool_put_pages(spool, chg);
>  		return ret;
>  	}
> 
> @@ -2934,12 +3014,13 @@ void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed)
>  {
>  	struct hstate *h = hstate_inode(inode);
>  	long chg = region_truncate(&inode->i_mapping->private_list, offset);
> +	struct hugepage_subpool *spool = subpool_inode(inode);
> 
>  	spin_lock(&inode->i_lock);
>  	inode->i_blocks -= (blocks_per_huge_page(h) * freed);
>  	spin_unlock(&inode->i_lock);
> 
> -	hugetlb_put_quota(inode->i_mapping, (chg - freed));
> +	hugepage_subpool_put_pages(spool, (chg - freed));
>  	hugetlb_acct_memory(h, -(chg - freed));
>  }
> 

-aneesh

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