locks: fix TOCTOU race when granting write lease

[ Upstream commit d6da19c9cace63290ccfccb1fc35151ffefc0bec ]

Thread A trying to acquire a write lease checks the value of i_readcount
and i_writecount in check_conflicting_open() to verify that its own fd
is the only fd referencing the file.

Thread B trying to open the file for read will call break_lease() in
do_dentry_open() before incrementing i_readcount, which leaves a small
window where thread A can acquire the write lease and then thread B
completes the open of the file for read without breaking the write lease
that was acquired by thread A.

Fix this race by incrementing i_readcount before checking for existing
leases, same as the case with i_writecount.

Use a helper put_file_access() to decrement i_readcount or i_writecount
in do_dentry_open() and __fput().

Fixes: 387e3746d01c ("locks: eliminate false positive conflicts for write lease")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

3 files changed, 15 insertions, 13 deletions

diff --git a/fs/file_table.c b/fs/file_table.c
index e8c9016703ad..6f297f9782fc 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -284,12 +284,7 @@ static void __fput(struct file *file)
 	}
 	fops_put(file->f_op);
 	put_pid(file->f_owner.pid);
-	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
-		i_readcount_dec(inode);
-	if (mode & FMODE_WRITER) {
-		put_write_access(inode);
-		__mnt_drop_write(mnt);
-	}
+	put_file_access(file);
 	dput(dentry);
 	if (unlikely(mode & FMODE_NEED_UNMOUNT))
 		dissolve_on_fput(mnt);
diff --git a/fs/internal.h b/fs/internal.h
index 4f1fe6d08866..9075490f21a6 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -100,6 +100,16 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
 extern struct file *alloc_empty_file(int, const struct cred *);
 extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
 
+static inline void put_file_access(struct file *file)
+{
+	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+		i_readcount_dec(file->f_inode);
+	} else if (file->f_mode & FMODE_WRITER) {
+		put_write_access(file->f_inode);
+		__mnt_drop_write(file->f_path.mnt);
+	}
+}
+
 /*
  * super.c
  */
diff --git a/fs/open.c b/fs/open.c
index 1ba1d2ab2ef0..5e322f188e83 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -786,7 +786,9 @@ static int do_dentry_open(struct file *f,
 		return 0;
 	}
 
-	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
+	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+		i_readcount_inc(inode);
+	} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = get_write_access(inode);
 		if (unlikely(error))
 			goto cleanup_file;
@@ -826,8 +828,6 @@ static int do_dentry_open(struct file *f,
 			goto cleanup_all;
 	}
 	f->f_mode |= FMODE_OPENED;
-	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
-		i_readcount_inc(inode);
 	if ((f->f_mode & FMODE_READ) &&
 	     likely(f->f_op->read || f->f_op->read_iter))
 		f->f_mode |= FMODE_CAN_READ;
@@ -880,10 +880,7 @@ cleanup_all:
 	if (WARN_ON_ONCE(error > 0))
 		error = -EINVAL;
 	fops_put(f->f_op);
-	if (f->f_mode & FMODE_WRITER) {
-		put_write_access(inode);
-		__mnt_drop_write(f->f_path.mnt);
-	}
+	put_file_access(f);
 cleanup_file:
 	path_put(&f->f_path);
 	f->f_path.mnt = NULL;