On Mon, Aug 15, 2022 at 2:21 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> On Sun, 2022-08-14 at 18:23 +0300, Amir Goldstein wrote:
> > 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.
> >
>
> Nice catch.
>
> > 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@xxxxxxxxx>
> > ---
> >
> > 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 *);
>
> Looks good to me. I like the new helper.
>
> In addition to Al's comment about which header this should go in, it
> might also be good to put a kerneldoc comment over it.
>
I'm sorry, I couldn't come up with a good description of
this arbitrary helper and I don't think this is so important
for an internal helper like this one.
For now, I will post without the kerneldoc.
If you disagree, please provide a description.
Thanks,
Amir.
