Re: [PATCH v2 0/29] block: Make blkdev_get_by_*() return handle

Christoph Hellwig
2023-08-28T14:28:33Z

On Sat, Aug 26, 2023 at 03:28:52AM +0100, Al Viro wrote:
> I mean, look at claim_swapfile() for example:
> p->bdev = blkdev_get_by_dev(inode->i_rdev,
> FMODE_READ | FMODE_WRITE | FMODE_EXCL, p);
> if (IS_ERR(p->bdev)) {
> error = PTR_ERR(p->bdev);
> p->bdev = NULL;
> return error;
> }
> p->old_block_size = block_size(p->bdev);
> error = set_blocksize(p->bdev, PAGE_SIZE);
> if (error < 0)
> return error;
> we already have the file opened, and we keep it opened all the way until
> the swapoff(2); here we have noticed that it's a block device and we
> * open the fucker again (by device number), this time claiming
> it with our swap_info_struct as holder, to be closed at swapoff(2) time
> (just before we close the file)

Note that some drivers look at FMODE_EXCL/BLK_OPEN_EXCL in ->open.
These are probably bogus and maybe we want to kill them, but that will
need an audit first.

> BTW, what happens if two threads call ioctl(fd, BLKBSZSET, &n)
> for the same descriptor that happens to have been opened O_EXCL?
> Without O_EXCL they would've been unable to claim the sucker at the same
> time - the holder we are using is the address of a function argument,
> i.e. something that points to kernel stack of the caller. Those would
> conflict and we either get set_blocksize() calls fully serialized, or
> one of the callers would eat -EBUSY. Not so in "opened with O_EXCL"
> case - they can very well overlap and IIRC set_blocksize() does *not*
> expect that kind of crap... It's all under CAP_SYS_ADMIN, so it's not
> as if it was a meaningful security hole anyway, but it does look fishy.

The user get to keep the pieces.. BLKBSZSET is kinda bogus anyway
as the soft blocksize only matters for buffer_head-like I/O, and
there only for file systems. Not idea why anyone would set it manually.