[PATCH v3] locks: fix TOCTOU race when granting write lease

[Amir Goldstein <amir73il@gmail.com>]

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>
---

Hi Al,

Added Jeff's RVB assuming you will be taking this patch.

Thanks,
Amir.

Changes since v2:
- Add RVB Jeff

Changes since v1:
- Move helper to internal.h

 fs/file_table.c |  7 +------
 fs/internal.h   | 10 ++++++++++
 fs/open.c       | 11 ++++-------
 3 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index 99c6796c9f28..dd88701e54a9 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -324,12 +324,7 @@ 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 87e96b9024ce..31861e6c3eff 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -101,6 +101,16 @@ 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 8a813fa5ca56..a98572585815 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -840,7 +840,9 @@ 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;
@@ -880,8 +882,6 @@ 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;
@@ -935,10 +935,7 @@ static int do_dentry_open(struct file *f,
 	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;
-- 
2.25.1