[PATCH] 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")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Hi Jeff,

This fixes a race I found during code audit - I do not have a reproducer
for it.

I ran the fstests I found for locks and leases:
generic/131 generic/478 generic/504 generic/571
and the LTP fcntl tests.

Encountered this warning with generic/131, but I also see it on
current master:

 =============================
 WARNING: suspicious RCU usage
 5.19.0-xfstests-14277-gbd6ab3ef4e93 #966 Not tainted
 -----------------------------
 include/net/sock.h:592 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:


 rcu_scheduler_active = 2, debug_locks = 1
 5 locks held by locktest/3996:
  #0: ffff88800be1d7a0 (&sb->s_type->i_mutex_key#8){+.+.}-{3:3}, at: __sock_release+0x25/0x97
  #1: ffff88800909ce00 (sk_lock-AF_INET){+.+.}-{0:0}, at: tcp_close+0x14/0x60
  #2: ffff888006847cc8 (&h->lhash2[i].lock){+.+.}-{2:2}, at: inet_unhash+0x3a/0xcf
  #3: ffffffff82a8ac18 (reuseport_lock){+...}-{2:2}, at: reuseport_detach_sock+0x17/0xb8
  #4: ffff88800909d0b0 (clock-AF_INET){++..}-{2:2}, at: bpf_sk_reuseport_detach+0x1b/0x85

 stack backtrace:
 CPU: 1 PID: 3996 Comm: locktest Not tainted 5.19.0-xfstests-14277-gbd6ab3ef4e93 #966
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
 Call Trace:
  <TASK>
  dump_stack_lvl+0x45/0x5d
  bpf_sk_reuseport_detach+0x5c/0x85
  reuseport_detach_sock+0x65/0xb8
  inet_unhash+0x55/0xcf
  tcp_set_state+0xb3/0x10d
  ? mark_lock.part.0+0x30/0x101
  __tcp_close+0x26/0x32d
  tcp_close+0x20/0x60
  inet_release+0x50/0x64
  __sock_release+0x32/0x97
  sock_close+0x14/0x1b
  __fput+0x118/0x1eb


Let me know what you think.

Thanks,
Amir.

 fs/file_table.c    |  7 +------
 fs/open.c          | 11 ++++-------
 include/linux/fs.h | 10 ++++++++++
 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/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;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9eced4cc286e..8bc04852c3da 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3000,6 +3000,16 @@ static inline void i_readcount_inc(struct inode *inode)
 	return;
 }
 #endif
+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);
+	}
+}
+
 extern int do_pipe_flags(int *, int);
 
 extern ssize_t kernel_read(struct file *, void *, size_t, loff_t *);
-- 
2.25.1