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Subject[PATCH v2 1/1] hugepages: Fix race between hugetlbfs umount and quota update.
This patch fixes a use-after-free problem in free_huge_page, with a quota update
happening after hugetlbfs umount. The problem results when a device driver,
which has mapped a hugepage, does a put_page. Put_page, calls free_huge_page,
which does a hugetlb_put_quota. As written, hugetlb_put_quota takes an
address_space struct pointer "mapping" as an argument. If the put_page occurs
after the hugetlbfs filesystem is unmounted, mapping points to freed memory.

Hugetlb_put_quota doesn't need the address_space pointer, it only uses it to
find the quota-record. Rather than an address-space struct pointer, this patched
code puts a hugetlbfs_sb_info struct pointer into page_private of the page
struct. In order to prevent the quota record from being freed at unmount, a
reference count and an active bit are added to the hugetlbfs_sb_info struct; the
reference count is increased by hugetlb_get_quota and decreased by
hugetlb_put_quota. When hugetlbfs is unmounted, it frees the hugetlbfs_sb_info
struct, but only if the reference count is zero. If the reference count is not
zero, it clears the active bit, and the last hugetlb_put_quota then frees the
hugetlbfs_sb_info struct.

Discussion was titled: Fix refcounting in hugetlbfs quota handling.
See: https://lkml.org/lkml/2011/8/11/28
See also: http://marc.info/?l=linux-mm&m=126928970510627&w=1

Version 2: Adding better description of how the race happens, removing
unnecessary inode->i_mapping->host->i_sb indirection when inode->i_sb has the
same meaning.

Signed-off-by: Andrew Barry <abarry@cray.com>
Cc: 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>

---


fs/hugetlbfs/inode.c | 40 ++++++++++++++++++++++++++--------------
include/linux/hugetlb.h | 9 +++++++--
mm/hugetlb.c | 23 ++++++++++++-----------
3 files changed, 45 insertions(+), 27 deletions(-)



diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 87b6e04..2ed1cca 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -615,8 +615,12 @@ static void hugetlbfs_put_super(struct s
struct hugetlbfs_sb_info *sbi = HUGETLBFS_SB(sb);

if (sbi) {
+ sbi->active = HPAGE_INACTIVE;
sb->s_fs_info = NULL;
- kfree(sbi);
+
+ /*Free only if used quota is zero. */
+ if (sbi->used_blocks == 0)
+ kfree(sbi);
}
}

@@ -851,6 +855,8 @@ hugetlbfs_fill_super(struct super_block
sbinfo->free_blocks = config.nr_blocks;
sbinfo->max_inodes = config.nr_inodes;
sbinfo->free_inodes = config.nr_inodes;
+ sbinfo->used_blocks = 0;
+ sbinfo->active = HPAGE_ACTIVE;
sb->s_maxbytes = MAX_LFS_FILESIZE;
sb->s_blocksize = huge_page_size(config.hstate);
sb->s_blocksize_bits = huge_page_shift(config.hstate);
@@ -874,30 +880,36 @@ out_free:
return -ENOMEM;
}

-int hugetlb_get_quota(struct address_space *mapping, long delta)
+int hugetlb_get_quota(struct hugetlbfs_sb_info *sbinfo, 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)
+ spin_lock(&sbinfo->stat_lock);
+ if ((sbinfo->free_blocks == -1) || (sbinfo->free_blocks - delta >= 0)) {
+ if (sbinfo->free_blocks != -1)
sbinfo->free_blocks -= delta;
- else
- ret = -ENOMEM;
- spin_unlock(&sbinfo->stat_lock);
+ sbinfo->used_blocks += delta;
+ sbinfo->active = HPAGE_ACTIVE;
+ } else {
+ ret = -ENOMEM;
}
+ spin_unlock(&sbinfo->stat_lock);

return ret;
}

-void hugetlb_put_quota(struct address_space *mapping, long delta)
+void hugetlb_put_quota(struct hugetlbfs_sb_info *sbinfo, long delta)
{
- struct hugetlbfs_sb_info *sbinfo = HUGETLBFS_SB(mapping->host->i_sb);
-
- if (sbinfo->free_blocks > -1) {
- spin_lock(&sbinfo->stat_lock);
+ spin_lock(&sbinfo->stat_lock);
+ if (sbinfo->free_blocks > -1)
sbinfo->free_blocks += delta;
+ sbinfo->used_blocks -= delta;
+ /* If hugetlbfs_put_super couldn't free sbinfo due to
+ * an outstanding quota reference, free it now. */
+ if ((sbinfo->used_blocks == 0) && (sbinfo->active == HPAGE_INACTIVE)) {
+ spin_unlock(&sbinfo->stat_lock);
+ kfree(sbinfo);
+ } else {
spin_unlock(&sbinfo->stat_lock);
}
}
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 19644e0..8780a91 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -142,11 +142,16 @@ struct hugetlbfs_config {
struct hstate *hstate;
};

+#define HPAGE_INACTIVE 0
+#define HPAGE_ACTIVE 1
+
struct hugetlbfs_sb_info {
long max_blocks; /* blocks allowed */
long free_blocks; /* blocks free */
long max_inodes; /* inodes allowed */
long free_inodes; /* inodes free */
+ long used_blocks; /* blocks used */
+ long active; /* active bit */
spinlock_t stat_lock;
struct hstate *hstate;
};
@@ -171,8 +176,8 @@ extern const struct file_operations huge
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);
+int hugetlb_get_quota(struct hugetlbfs_sb_info *sbinfo, long delta);
+void hugetlb_put_quota(struct hugetlbfs_sb_info *sbinfo, long delta);

static inline int is_file_hugepages(struct file *file)
{
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dae27ba..0f39cde 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -533,9 +533,9 @@ static void free_huge_page(struct page *
*/
struct hstate *h = page_hstate(page);
int nid = page_to_nid(page);
- struct address_space *mapping;
+ struct hugetlbfs_sb_info *sbinfo;

- mapping = (struct address_space *) page_private(page);
+ sbinfo = ( struct hugetlbfs_sb_info *) page_private(page);
set_page_private(page, 0);
page->mapping = NULL;
BUG_ON(page_count(page));
@@ -551,8 +551,8 @@ static void free_huge_page(struct page *
enqueue_huge_page(h, page);
}
spin_unlock(&hugetlb_lock);
- if (mapping)
- hugetlb_put_quota(mapping, 1);
+ if (sbinfo)
+ hugetlb_put_quota(sbinfo, 1);
}

static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
@@ -1035,7 +1035,7 @@ static struct page *alloc_huge_page(stru
if (chg < 0)
return ERR_PTR(-VM_FAULT_OOM);
if (chg)
- if (hugetlb_get_quota(inode->i_mapping, chg))
+ if (hugetlb_get_quota(HUGETLBFS_SB(inode->i_sb), chg))
return ERR_PTR(-VM_FAULT_SIGBUS);

spin_lock(&hugetlb_lock);
@@ -1045,12 +1045,12 @@ static struct page *alloc_huge_page(stru
if (!page) {
page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
if (!page) {
- hugetlb_put_quota(inode->i_mapping, chg);
+ hugetlb_put_quota(HUGETLBFS_SB(inode->i_sb), chg);
return ERR_PTR(-VM_FAULT_SIGBUS);
}
}

- set_page_private(page, (unsigned long) mapping);
+ set_page_private(page, (unsigned long) HUGETLBFS_SB(inode->i_sb));

vma_commit_reservation(h, vma, addr);

@@ -2086,7 +2086,8 @@ static void hugetlb_vm_op_close(struct v

if (reserve) {
hugetlb_acct_memory(h, -reserve);
- hugetlb_put_quota(vma->vm_file->f_mapping, reserve);
+ hugetlb_put_quota(HUGETLBFS_SB(
+ vma->vm_file->f_mapping->host->i_sb), reserve);
}
}
}
@@ -2884,7 +2885,7 @@ int hugetlb_reserve_pages(struct inode *
return chg;

/* There must be enough filesystem quota for the mapping */
- if (hugetlb_get_quota(inode->i_mapping, chg))
+ if (hugetlb_get_quota(HUGETLBFS_SB(inode->i_sb), chg))
return -ENOSPC;

/*
@@ -2893,7 +2894,7 @@ int hugetlb_reserve_pages(struct inode *
*/
ret = hugetlb_acct_memory(h, chg);
if (ret < 0) {
- hugetlb_put_quota(inode->i_mapping, chg);
+ hugetlb_put_quota(HUGETLBFS_SB(inode->i_sb), chg);
return ret;
}

@@ -2922,7 +2923,7 @@ void hugetlb_unreserve_pages(struct inod
inode->i_blocks -= (blocks_per_huge_page(h) * freed);
spin_unlock(&inode->i_lock);

- hugetlb_put_quota(inode->i_mapping, (chg - freed));
+ hugetlb_put_quota(HUGETLBFS_SB(inode->i_sb), (chg - freed));
hugetlb_acct_memory(h, -(chg - freed));
}


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