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.
Al, did you want to take this via your tree or do you want me to take it
via the filelocks tree?
Thanks,
--
Jeff Layton <jlayton@xxxxxxxxxx>
