fuse: hotfix truncate_pagecache() issue

The way how fuse calls truncate_pagecache() from fuse_change_attributes()
is completely wrong. Because, w/o i_mutex held, we never sure whether
'oldsize' and 'attr->size' are valid by the time of execution of
truncate_pagecache(inode, oldsize, attr->size). In fact, as soon as we
released fc->lock in the middle of fuse_change_attributes(), we completely
loose control of actions which may happen with given inode until we reach
truncate_pagecache. The list of potentially dangerous actions includes
mmap-ed reads and writes, ftruncate(2) and write(2) extending file size.

The typical outcome of doing truncate_pagecache() with outdated arguments
is data corruption from user point of view. This is (in some sense)
acceptable in cases when the issue is triggered by a change of the file on
the server (i.e. externally wrt fuse operation), but it is absolutely
intolerable in scenarios when a single fuse client modifies a file without
any external intervention. A real life case I discovered by fsx-linux
looked like this:

1. Shrinking ftruncate(2) comes to fuse_do_setattr(). The latter sends
FUSE_SETATTR to the server synchronously, but before getting fc->lock ...
2. fuse_dentry_revalidate() is asynchronously called. It sends FUSE_LOOKUP
to the server synchronously, then calls fuse_change_attributes(). The
latter updates i_size, releases fc->lock, but before comparing oldsize vs
attr->size..
3. fuse_do_setattr() from the first step proceeds by acquiring fc->lock and
updating attributes and i_size, but now oldsize is equal to
outarg.attr.size because i_size has just been updated (step 2). Hence,
fuse_do_setattr() returns w/o calling truncate_pagecache().
4. As soon as ftruncate(2) completes, the user extends file size by
write(2) making a hole in the middle of file, then reads data from the hole
either by read(2) or mmap-ed read. The user expects to get zero data from
the hole, but gets stale data because truncate_pagecache() is not executed
yet.

The scenario above illustrates one side of the problem: not truncating the
page cache even though we should. Another side corresponds to truncating
page cache too late, when the state of inode changed significantly.
Theoretically, the following is possible:

1. As in the previous scenario fuse_dentry_revalidate() discovered that
i_size changed (due to our own fuse_do_setattr()) and is going to call
truncate_pagecache() for some 'new_size' it believes valid right now. But
by the time that particular truncate_pagecache() is called ...
2. fuse_do_setattr() returns (either having called truncate_pagecache() or
not -- it doesn't matter).
3. The file is extended either by write(2) or ftruncate(2) or fallocate(2).
4. mmap-ed write makes a page in the extended region dirty.

The result will be the lost of data user wrote on the fourth step.

The patch is a hotfix resolving the issue in a simplistic way: let's skip
dangerous i_size update and truncate_pagecache if an operation changing
file size is in progress. This simplistic approach looks correct for the
cases w/o external changes. And to handle them properly, more sophisticated
and intrusive techniques (e.g. NFS-like one) would be required. I'd like to
postpone it until the issue is well discussed on the mailing list(s).

Changed in v2:
 - improved patch description to cover both sides of the issue.

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@vger.kernel.org

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d1715b30f6c..d409deafc67 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -629,7 +629,8 @@ static void fuse_read_update_size(struct inode *inode, loff_t size,
 	struct fuse_inode *fi = get_fuse_inode(inode);
 
 	spin_lock(&fc->lock);
-	if (attr_ver == fi->attr_version && size < inode->i_size) {
+	if (attr_ver == fi->attr_version && size < inode->i_size &&
+	    !test_bit(FUSE_I_SIZE_UNSTABLE, &fi->state)) {
 		fi->attr_version = ++fc->attr_version;
 		i_size_write(inode, size);
 	}
@@ -1032,12 +1033,16 @@ static ssize_t fuse_perform_write(struct file *file,
 {
 	struct inode *inode = mapping->host;
 	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 	int err = 0;
 	ssize_t res = 0;
 
 	if (is_bad_inode(inode))
 		return -EIO;
 
+	if (inode->i_size < pos + iov_iter_count(ii))
+		set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
+
 	do {
 		struct fuse_req *req;
 		ssize_t count;
@@ -1073,6 +1078,7 @@ static ssize_t fuse_perform_write(struct file *file,
 	if (res > 0)
 		fuse_write_update_size(inode, pos);
 
+	clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
 	fuse_invalidate_attr(inode);
 
 	return res > 0 ? res : err;