From 387e3746d01c34457d6a73688acd90428725070b Mon Sep 17 00:00:00 2001 From: Amir Goldstein Date: Fri, 7 Jun 2019 17:24:38 +0300 Subject: locks: eliminate false positive conflicts for write lease check_conflicting_open() is checking for existing fd's open for read or for write before allowing to take a write lease. The check that was implemented using i_count and d_count is an approximation that has several false positives. For example, overlayfs since v4.19, takes an extra reference on the dentry; An open with O_PATH takes a reference on the dentry although the file cannot be read nor written. Change the implementation to use i_readcount and i_writecount to eliminate the false positive conflicts and allow a write lease to be taken on an overlayfs file. The change of behavior with existing fd's open with O_PATH is symmetric w.r.t. current behavior of lease breakers - an open with O_PATH currently does not break a write lease. This increases the size of struct inode by 4 bytes on 32bit archs when CONFIG_FILE_LOCKING is defined and CONFIG_IMA was not already defined. Signed-off-by: Amir Goldstein Signed-off-by: Jeff Layton --- fs/locks.c | 42 +++++++++++++++++++++++++++--------------- 1 file changed, 27 insertions(+), 15 deletions(-) (limited to 'fs/locks.c') diff --git a/fs/locks.c b/fs/locks.c index 0cc2b9f30e22..de87a3231789 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -1763,10 +1763,10 @@ int fcntl_getlease(struct file *filp) } /** - * check_conflicting_open - see if the given dentry points to a file that has + * check_conflicting_open - see if the given file points to an inode that has * an existing open that would conflict with the * desired lease. - * @dentry: dentry to check + * @filp: file to check * @arg: type of lease that we're trying to acquire * @flags: current lock flags * @@ -1774,30 +1774,42 @@ int fcntl_getlease(struct file *filp) * conflict with the lease we're trying to set. */ static int -check_conflicting_open(const struct dentry *dentry, const long arg, int flags) +check_conflicting_open(struct file *filp, const long arg, int flags) { - int ret = 0; - struct inode *inode = dentry->d_inode; + struct inode *inode = locks_inode(filp); + int self_wcount = 0, self_rcount = 0; if (flags & FL_LAYOUT) return 0; - if ((arg == F_RDLCK) && inode_is_open_for_write(inode)) - return -EAGAIN; + if (arg == F_RDLCK) + return inode_is_open_for_write(inode) ? -EAGAIN : 0; + else if (arg != F_WRLCK) + return 0; + + /* + * Make sure that only read/write count is from lease requestor. + * Note that this will result in denying write leases when i_writecount + * is negative, which is what we want. (We shouldn't grant write leases + * on files open for execution.) + */ + if (filp->f_mode & FMODE_WRITE) + self_wcount = 1; + else if (filp->f_mode & FMODE_READ) + self_rcount = 1; - if ((arg == F_WRLCK) && ((d_count(dentry) > 1) || - (atomic_read(&inode->i_count) > 1))) - ret = -EAGAIN; + if (atomic_read(&inode->i_writecount) != self_wcount || + atomic_read(&inode->i_readcount) != self_rcount) + return -EAGAIN; - return ret; + return 0; } static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **priv) { struct file_lock *fl, *my_fl = NULL, *lease; - struct dentry *dentry = filp->f_path.dentry; - struct inode *inode = dentry->d_inode; + struct inode *inode = locks_inode(filp); struct file_lock_context *ctx; bool is_deleg = (*flp)->fl_flags & FL_DELEG; int error; @@ -1832,7 +1844,7 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr percpu_down_read(&file_rwsem); spin_lock(&ctx->flc_lock); time_out_leases(inode, &dispose); - error = check_conflicting_open(dentry, arg, lease->fl_flags); + error = check_conflicting_open(filp, arg, lease->fl_flags); if (error) goto out; @@ -1889,7 +1901,7 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr * precedes these checks. */ smp_mb(); - error = check_conflicting_open(dentry, arg, lease->fl_flags); + error = check_conflicting_open(filp, arg, lease->fl_flags); if (error) { locks_unlink_lock_ctx(lease); goto out; -- cgit v1.2.3-59-g8ed1b