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block: handle BLK_OPEN_RESTRICT_WRITES correctly

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Last kernel release we introduce CONFIG_BLK_DEV_WRITE_MOUNTED. By
default this option is set. When it is set the long-standing behavior
of being able to write to mounted block devices is enabled.

But in order to guard against unintended corruption by writing to the
block device buffer cache CONFIG_BLK_DEV_WRITE_MOUNTED can be turned
off. In that case it isn't possible to write to mounted block devices
anymore.

A filesystem may open its block devices with BLK_OPEN_RESTRICT_WRITES
which disallows concurrent BLK_OPEN_WRITE access. When we still had the
bdev handle around we could recognize BLK_OPEN_RESTRICT_WRITES because
the mode was passed around. Since we managed to get rid of the bdev
handle we changed that logic to recognize BLK_OPEN_RESTRICT_WRITES based
on whether the file was opened writable and writes to that block device
are blocked. That logic doesn't work because we do allow
BLK_OPEN_RESTRICT_WRITES to be specified without BLK_OPEN_WRITE.

Fix the detection logic and use an FMODE_* bit. We could've also abused
O_EXCL as an indicator that BLK_OPEN_RESTRICT_WRITES has been requested.
For userspace open paths O_EXCL will never be retained but for internal
opens where we open files that are never installed into a file
descriptor table this is fine. But it would be a gamble that this
doesn't cause bugs. Note that BLK_OPEN_RESTRICT_WRITES is an internal
only flag that cannot directly be raised by userspace. It is implicitly
raised during mounting.

Passes xftests and blktests with CONFIG_BLK_DEV_WRITE_MOUNTED set and
unset.

Link: https://lore.kernel.org/r/ZfyyEwu9Uq5Pgb94@casper.infradead.org
Link: https://lore.kernel.org/r/20240323-zielbereich-mittragen-6fdf14876c3e@brauner
Fixes: 321de651fa56 ("block: don't rely on BLK_OPEN_RESTRICT_WRITES when yielding write access")
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2024-03-23 17:11:19 +01:00
parent 4cece76496
commit ddd65e19c6
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
2 changed files with 9 additions and 7 deletions
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14
block/bdev.c
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@ -821,13 +821,11 @@ static void bdev_yield_write_access(struct file *bdev_file)
return;
bdev = file_bdev(bdev_file);
/* Yield exclusive or shared write access. */
if (bdev_file->f_mode & FMODE_WRITE) {
if (bdev_writes_blocked(bdev))
bdev_unblock_writes(bdev);
else
bdev->bd_writers--;
}
if (bdev_file->f_mode & FMODE_WRITE_RESTRICTED)
bdev_unblock_writes(bdev);
else if (bdev_file->f_mode & FMODE_WRITE)
bdev->bd_writers--;
}
/**
@ -907,6 +905,8 @@ int bdev_open(struct block_device *bdev, blk_mode_t mode, void *holder,
bdev_file->f_mode |= FMODE_BUF_RASYNC | FMODE_CAN_ODIRECT;
if (bdev_nowait(bdev))
bdev_file->f_mode |= FMODE_NOWAIT;
if (mode & BLK_OPEN_RESTRICT_WRITES)
bdev_file->f_mode |= FMODE_WRITE_RESTRICTED;
bdev_file->f_mapping = bdev->bd_inode->i_mapping;
bdev_file->f_wb_err = filemap_sample_wb_err(bdev_file->f_mapping);
bdev_file->private_data = holder;

2
include/linux/fs.h
View File

@ -121,6 +121,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
#define FMODE_PWRITE ((__force fmode_t)0x10)
/* File is opened for execution with sys_execve / sys_uselib */
#define FMODE_EXEC ((__force fmode_t)0x20)
/* File writes are restricted (block device specific) */
#define FMODE_WRITE_RESTRICTED ((__force fmode_t)0x40)
/* 32bit hashes as llseek() offset (for directories) */
#define FMODE_32BITHASH ((__force fmode_t)0x200)
/* 64bit hashes as llseek() offset (for directories) */