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    SubjectRe: [PATCH 2/2] hugepages: Fix use after free bug in "quota" handling
    From
    On Wed, Mar 7, 2012 at 12:48 PM, David Gibson
    <david@gibson.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@cray.com>
    > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    > Cc: Hugh Dickins <hughd@google.com>
    > Cc: Mel Gorman <mgorman@suse.de>
    > Cc: Minchan Kim <minchan.kim@gmail.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Hillf Danton <dhillf@gmail.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(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);
    > +
    > +       spin_unlock(&spool->lock);
    > +
    > +       /* 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)
    > +{
    > +       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)
    > +{
    > +       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);
    >
    > -       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);

    Like current code, quota is handed back *unconditionally*, but ...

    >  }
    >
    >  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);

    ... quota is allocated only if needed.

    Given that mismatch, are you fixing the use-after-free bug, or
    quota maintenance, or both?

    >
    >        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));
    >  }
    >
    > --
    > 1.7.9.1
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