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The typical outcome of doing truncate_pagecache() with outdated arguments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

4 files changed, 17 insertions, 3 deletions

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index c49b8c722e27..c8334f75c8c9 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1590,6 +1590,7 @@ int fuse_do_setattr(struct inode *inode, struct iattr *attr,
                     struct file *file)
 {
         struct fuse_conn *fc = get_fuse_conn(inode);
+        struct fuse_inode *fi = get_fuse_inode(inode);
         struct fuse_req *req;
         struct fuse_setattr_in inarg;
         struct fuse_attr_out outarg;
@@ -1617,8 +1618,10 @@ int fuse_do_setattr(struct inode *inode, struct iattr *attr,
         if (IS_ERR(req))
                 return PTR_ERR(req);
 
-        if (is_truncate)
+        if (is_truncate) {
                 fuse_set_nowrite(inode);
+                set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
+        }
 
         memset(&inarg, 0, sizeof(inarg));
         memset(&outarg, 0, sizeof(outarg));
@@ -1680,12 +1683,14 @@ int fuse_do_setattr(struct inode *inode, struct iattr *attr,
                 invalidate_inode_pages2(inode->i_mapping);
         }
 
+        clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
         return 0;
 
 error:
         if (is_truncate)
                 fuse_release_nowrite(inode);
 
+        clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
         return err;
 }
 
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d1715b30f6c4..d409deafc67b 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;
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index fde7249a3a96..5ced199b50bb 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -115,6 +115,8 @@ struct fuse_inode {
 enum {
         /** Advise readdirplus  */
         FUSE_I_ADVISE_RDPLUS,
+        /** An operation changing file size is in progress  */
+        FUSE_I_SIZE_UNSTABLE,
 };
 
 struct fuse_conn;
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 0b578598c6ac..e0fe703ee3d6 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -201,7 +201,8 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
         struct timespec old_mtime;
 
         spin_lock(&fc->lock);
-        if (attr_version != 0 && fi->attr_version > attr_version) {
+        if ((attr_version != 0 && fi->attr_version > attr_version) ||
+            test_bit(FUSE_I_SIZE_UNSTABLE, &fi->state)) {
                 spin_unlock(&fc->lock);
                 return;
         }