Re: [PATCH v2] locks: fix TOCTOU race when granting write lease

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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>




[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [NTFS 3]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [NTFS 3]     [Samba]     [Device Mapper]     [CEPH Development]

  Powered by Linux