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SubjectRe: [PATCH 2/2] hugepages: Fix use after free bug in "quota" handling
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On Wed,  7 Mar 2012 15:48:14 +1100, 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() 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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