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Subject[PATCH 2/6] fanotify: synchronize adding/removing a mark by means of group mutex

When removing flags from the mask of a mark the mask may go to 0 and the mark should
be destroyed. But right before we destroy it there is a race which allows a
concurrent process to add flags to the mask of the same marks that we are going to destroy.
Thus we might end up destroying a mark that has not all flags cleared in its mask.

This patch uses the group mutex to serialize the access to a mark, so that a concurrent
concurrent process can no longer access it if it is going to be destroyed.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
---
fs/notify/fanotify/fanotify_user.c | 60 ++++++++++++++++++++++++++---------
1 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 5d2e683..c9cd1b9 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -540,15 +540,21 @@ static int fanotify_remove_vfsmount_mark(struct fsnotify_group *group,
{
struct fsnotify_mark *fsn_mark = NULL;
__u32 removed;
+ int ret = 0;

+ mutex_lock(&group->mutex);
fsn_mark = fsnotify_find_vfsmount_mark(group, mnt);
- if (!fsn_mark)
- return -ENOENT;
+ if (!fsn_mark) {
+ ret = -ENOENT;
+ goto err;
+ }

removed = fanotify_mark_remove_from_mask(fsn_mark, mask, flags);
fsnotify_put_mark(fsn_mark);
if (removed & mnt->mnt_fsnotify_mask)
fsnotify_recalc_vfsmount_mask(mnt);
+err:
+ mutex_unlock(&group->mutex);

return 0;
}
@@ -559,16 +565,22 @@ static int fanotify_remove_inode_mark(struct fsnotify_group *group,
{
struct fsnotify_mark *fsn_mark = NULL;
__u32 removed;
+ int ret = 0;

+ mutex_lock(&group->mutex);
fsn_mark = fsnotify_find_inode_mark(group, inode);
- if (!fsn_mark)
- return -ENOENT;
+ if (!fsn_mark) {
+ ret = -ENOENT;
+ goto err;
+ }

removed = fanotify_mark_remove_from_mask(fsn_mark, mask, flags);
/* matches the fsnotify_find_inode_mark() */
fsnotify_put_mark(fsn_mark);
if (removed & inode->i_fsnotify_mask)
fsnotify_recalc_inode_mask(inode);
+err:
+ mutex_unlock(&group->mutex);

return 0;
}
@@ -608,26 +620,34 @@ static int fanotify_add_vfsmount_mark(struct fsnotify_group *group,
__u32 added;
int ret = 0;

+ mutex_lock(&group->mutex);
fsn_mark = fsnotify_find_vfsmount_mark(group, mnt);
if (!fsn_mark) {
- if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
- return -ENOSPC;
+ if (atomic_read(&group->num_marks) >
+ group->fanotify_data.max_marks) {
+ ret = -ENOSPC;
+ goto err;
+ }

fsn_mark = kmem_cache_alloc(fanotify_mark_cache, GFP_KERNEL);
- if (!fsn_mark)
- return -ENOMEM;
+ if (!fsn_mark) {
+ ret = -ENOMEM;
+ goto err;
+ }

fsnotify_init_mark(fsn_mark, fanotify_free_mark);
ret = fsnotify_add_mark(fsn_mark, group, NULL, mnt, 0);
if (ret)
- goto err;
+ goto err2;
}
added = fanotify_mark_add_to_mask(fsn_mark, mask, flags);

if (added & ~mnt->mnt_fsnotify_mask)
fsnotify_recalc_vfsmount_mask(mnt);
-err:
+err2:
fsnotify_put_mark(fsn_mark);
+err:
+ mutex_unlock(&group->mutex);
return ret;
}

@@ -651,26 +671,34 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
(atomic_read(&inode->i_writecount) > 0))
return 0;

+ mutex_lock(&group->mutex);
fsn_mark = fsnotify_find_inode_mark(group, inode);
if (!fsn_mark) {
- if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
- return -ENOSPC;
+ if (atomic_read(&group->num_marks) >
+ group->fanotify_data.max_marks) {
+ ret = -ENOSPC;
+ goto err;
+ }

fsn_mark = kmem_cache_alloc(fanotify_mark_cache, GFP_KERNEL);
- if (!fsn_mark)
- return -ENOMEM;
+ if (!fsn_mark) {
+ ret = -ENOMEM;
+ goto err;
+ }

fsnotify_init_mark(fsn_mark, fanotify_free_mark);
ret = fsnotify_add_mark(fsn_mark, group, inode, NULL, 0);
if (ret)
- goto err;
+ goto err2;
}
added = fanotify_mark_add_to_mask(fsn_mark, mask, flags);

if (added & ~inode->i_fsnotify_mask)
fsnotify_recalc_inode_mask(inode);
-err:
+err2:
fsnotify_put_mark(fsn_mark);
+err:
+ mutex_unlock(&group->mutex);
return ret;
}

--
1.5.6.5


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