On Tue, 2022-08-16 at 17:08 +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.
>
> 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 Al,
>
> Per your request, here is v2 with moved helper.
>
> Jeff has suggested that I add kerneldoc, but I couldn't
> come up with a good description and didn't think it was very
> important, so I left it as is.
>
It's not terribly important.
> Thanks,
> Amir.
>
> 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;
Let me know if you need me to take this via my tree. Since Al responded,
I'm assuming he'll pick this one up.
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
