[PATCH v4 1/2] cifs: prevent readdir from changing file size due to stale directory metadata

[Frank Sorenson <sorenson@redhat.com>]

Windows Server's directory enumeration metadata lags behind the actual
file size after a write+close or rename.  A concurrent readdir() in the
window between close() returning to userspace and stat() being called
overwrites the correct cached i_size with the stale server value, causing
stat() to return the wrong size.

Once _cifsFileInfo_put() removes the last writable handle from
openFileList, is_size_safe_to_change() permits readdir to overwrite
i_size.  smb2_close_getattr() then stamps cifs_i->time = jiffies, making
the corrupt cached value appear fresh to the next stat().

The existing check (see Fixes:) only blocked stale size updates while
an active RW lease was held, not after the last writable handle closes.

Add cifsInodeInfo->time_last_write, written via smp_store_release() at
writable close and on setattr/truncate.  is_size_safe_to_change() checks
is_inode_writable() first (acquiring open_file_lock), then rejects a
readdir size update if time_last_write falls within acregmax jiffies.
The spinlock release in _cifsFileInfo_put() forms a store-release barrier
that pairs with the spin_lock() (load-acquire) in is_inode_writable(),
ensuring the subsequent smp_load_acquire() on time_last_write observes
any update from a concurrent close().  When a size update is rejected and
the server value differs from the cached one, cifs_i->time is cleared to
force a fresh QUERY_INFO on the next stat().

readdir is also blocked from changing i_size while writable handles are
open or an RW lease is held, even on direct-IO mounts.

For deferred close (closetimeo > 0), time_last_write is refreshed at the
actual server close in smb2_deferred_work_close() and in the
cifs_close_deferred_file*() drain paths invoked by lease/oplock breaks
and tcon teardown, anchoring the protection window to the real close time
rather than the earlier userspace close.

time_last_write == 0 skips the time_before() check to avoid false
positives near boot on 32-bit systems where jiffies starts close to
INITIAL_JIFFIES.

Does not reproduce against Samba or with actimeo=0.

Fixes: e4b61f3b1c67 ("cifs: prevent updating file size from server if we have a read/write lease")
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
---
diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index 66b9104e7ca2..1788d93a2522 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -440,6 +440,7 @@ cifs_alloc_inode(struct super_block *sb)
 		return NULL;
 	cifs_inode->cifsAttrs = ATTR_ARCHIVE;	/* default */
 	cifs_inode->time = 0;
+	cifs_inode->time_last_write = 0;
 	/*
 	 * Until the file is open and we have gotten oplock info back from the
 	 * server, can not assume caching of file data or metadata.
diff --git a/fs/smb/client/cifsglob.h b/fs/smb/client/cifsglob.h
index 08e94633a9c1..79e4e84f8985 100644
--- a/fs/smb/client/cifsglob.h
+++ b/fs/smb/client/cifsglob.h
@@ -1566,6 +1566,7 @@ struct cifsInodeInfo {
 	spinlock_t writers_lock;
 	unsigned int writers;		/* Number of writers on this inode */
 	unsigned long time;		/* jiffies of last update of inode */
+	unsigned long time_last_write;	/* jiffies of last writable close or truncate */
 	u64  uniqueid;			/* server inode number */
 	u64  createtime;		/* creation time on server */
 	__u8 lease_key[SMB2_LEASE_KEY_SIZE];	/* lease key for this inode */
diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
index 968740e7c9c3..b279a44be729 100644
--- a/fs/smb/client/file.c
+++ b/fs/smb/client/file.c
@@ -1423,11 +1423,21 @@ void smb2_deferred_work_close(struct work_struct *work)
 {
 	struct cifsFileInfo *cfile = container_of(work,
 			struct cifsFileInfo, deferred.work);
+	struct cifsInodeInfo *cinode = CIFS_I(d_inode(cfile->dentry));
 
-	spin_lock(&CIFS_I(d_inode(cfile->dentry))->deferred_lock);
+	spin_lock(&cinode->deferred_lock);
 	cifs_del_deferred_close(cfile);
 	cfile->deferred_close_scheduled = false;
-	spin_unlock(&CIFS_I(d_inode(cfile->dentry))->deferred_lock);
+	spin_unlock(&cinode->deferred_lock);
+	/*
+	 * Refresh time_last_write immediately before the actual server close
+	 * so the protection window is anchored to the real close time, not
+	 * the earlier userspace close time stored by cifs_close().
+	 */
+	if (OPEN_FMODE(cfile->f_flags) & FMODE_WRITE) {
+		/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+		smp_store_release(&cinode->time_last_write, jiffies);
+	}
 	_cifsFileInfo_put(cfile, true, false);
 }
 
@@ -1457,6 +1467,10 @@ int cifs_close(struct inode *inode, struct file *file)
 	if (file->private_data != NULL) {
 		cfile = file->private_data;
 		file->private_data = NULL;
+		if (file->f_mode & FMODE_WRITE) {
+			/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+			smp_store_release(&cinode->time_last_write, jiffies);
+		}
 		dclose = kmalloc_obj(struct cifs_deferred_close);
 		if ((cfile->status_file_deleted == false) &&
 		    (smb2_can_defer_close(inode, dclose))) {
@@ -3225,13 +3239,26 @@ static int is_inode_writable(struct cifsInodeInfo *cifs_inode)
 bool is_size_safe_to_change(struct cifsInodeInfo *cifsInode, __u64 end_of_file,
 			    bool from_readdir)
 {
+	struct cifs_sb_info *cifs_sb;
+	unsigned long tlw;
+
 	if (!cifsInode)
 		return true;
 
+	cifs_sb = CIFS_SB(cifsInode);
+
 	if (is_inode_writable(cifsInode) ||
 		((cifsInode->oplock & CIFS_CACHE_RW_FLG) != 0 && from_readdir)) {
 		/* This inode is open for write at least once */
-		struct cifs_sb_info *cifs_sb = CIFS_SB(cifsInode);
+
+		/*
+		 * Readdir data is unreliable when we have writable handles or
+		 * an exclusive lease -- never allow it to change i_size, even
+		 * on direct-IO mounts where the server's directory metadata
+		 * can still lag behind the actual file state.
+		 */
+		if (from_readdir)
+			return false;
 
 		if (cifs_sb_flags(cifs_sb) & CIFS_MOUNT_DIRECT_IO) {
 			/* since no page cache to corrupt on directio
@@ -3243,8 +3270,40 @@ bool is_size_safe_to_change(struct cifsInodeInfo *cifsInode, __u64 end_of_file,
 			return true;
 
 		return false;
-	} else
-		return true;
+	}
+
+	/*
+	 * No writable handles open. Check whether we are within the attribute
+	 * cache validity window of a recent local modification.
+	 *
+	 * For the close() path: cifs_close() calls smp_store_release() on
+	 * time_last_write before _cifsFileInfo_put() removes the handle under
+	 * open_file_lock. That spin_unlock() is a store-release that pairs
+	 * with the spin_lock() (load-acquire) in is_inode_writable() above,
+	 * so if is_inode_writable() returned false the smp_load_acquire()
+	 * below is guaranteed to observe any time_last_write update from a
+	 * concurrent close().
+	 *
+	 * For the setattr/truncate paths: those callers use smp_store_release()
+	 * directly; the smp_load_acquire() below pairs with that store. There
+	 * is no shared lock between setattr and readdir, so this relies on
+	 * acquire-release semantics alone. The store propagation latency on
+	 * weakly-ordered architectures (nanoseconds) is negligible relative to
+	 * the acregmax window (seconds) and the readdir RPC round-trip
+	 * (milliseconds), making this a sound design choice in practice.
+	 *
+	 * time_last_write == 0 means the inode has never been written locally;
+	 * skip the window check to avoid false positives near boot time when
+	 * jiffies is still close to INITIAL_JIFFIES on 32-bit systems.
+	 */
+	if (from_readdir) {
+		/* Pairs with smp_store_release() at close and truncate sites. */
+		tlw = smp_load_acquire(&cifsInode->time_last_write);
+		if (tlw && time_before(jiffies, tlw + cifs_sb->ctx->acregmax))
+			return false;
+	}
+
+	return true;
 }
 
 void cifs_oplock_break(struct work_struct *work)
diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c
index deed04dd9b91..b2806371bfde 100644
--- a/fs/smb/client/inode.c
+++ b/fs/smb/client/inode.c
@@ -237,6 +237,8 @@ cifs_fattr_to_inode(struct inode *inode, struct cifs_fattr *fattr,
 	if (is_size_safe_to_change(cifs_i, fattr->cf_eof, from_readdir)) {
 		i_size_write(inode, fattr->cf_eof);
 		inode->i_blocks = CIFS_INO_BLOCKS(fattr->cf_bytes);
+	} else if (from_readdir && i_size_read(inode) != fattr->cf_eof) {
+		cifs_i->time = 0;
 	}
 
 	if (S_ISLNK(fattr->cf_mode) && fattr->cf_symlink_target) {
@@ -3277,6 +3279,8 @@ cifs_setattr_unix(struct dentry *direntry, struct iattr *attrs)
 
 	if ((attrs->ia_valid & ATTR_SIZE) &&
 	    attrs->ia_size != i_size_read(inode)) {
+		/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+		smp_store_release(&cifsInode->time_last_write, jiffies);
 		truncate_setsize(inode, attrs->ia_size);
 		netfs_resize_file(&cifsInode->netfs, attrs->ia_size, true);
 		fscache_resize_cookie(cifs_inode_cookie(inode), attrs->ia_size);
@@ -3478,6 +3482,8 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs)
 
 	if ((attrs->ia_valid & ATTR_SIZE) &&
 	    attrs->ia_size != i_size_read(inode)) {
+		/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+		smp_store_release(&cifsInode->time_last_write, jiffies);
 		truncate_setsize(inode, attrs->ia_size);
 		netfs_resize_file(&cifsInode->netfs, attrs->ia_size, true);
 		fscache_resize_cookie(cifs_inode_cookie(inode), attrs->ia_size);
diff --git a/fs/smb/client/misc.c b/fs/smb/client/misc.c
index e4bac2a0b85d..0301ca932ce9 100644
--- a/fs/smb/client/misc.c
+++ b/fs/smb/client/misc.c
@@ -524,7 +524,14 @@ cifs_close_deferred_file(struct cifsInodeInfo *cifs_inode)
 	spin_unlock(&cifs_inode->open_file_lock);
 
 	list_for_each_entry_safe(tmp_list, tmp_next_list, &file_head, list) {
-		_cifsFileInfo_put(tmp_list->cfile, false, false);
+		struct cifsFileInfo *cfile = tmp_list->cfile;
+
+		if (OPEN_FMODE(cfile->f_flags) & FMODE_WRITE) {
+			/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+			smp_store_release(&CIFS_I(d_inode(cfile->dentry))->time_last_write,
+					  jiffies);
+		}
+		_cifsFileInfo_put(cfile, false, false);
 		list_del(&tmp_list->list);
 		kfree(tmp_list);
 	}
@@ -557,7 +564,14 @@ cifs_close_all_deferred_files(struct cifs_tcon *tcon)
 	spin_unlock(&tcon->open_file_lock);
 
 	list_for_each_entry_safe(tmp_list, tmp_next_list, &file_head, list) {
-		_cifsFileInfo_put(tmp_list->cfile, true, false);
+		struct cifsFileInfo *cfile = tmp_list->cfile;
+
+		if (OPEN_FMODE(cfile->f_flags) & FMODE_WRITE) {
+			/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+			smp_store_release(&CIFS_I(d_inode(cfile->dentry))->time_last_write,
+					  jiffies);
+		}
+		_cifsFileInfo_put(cfile, true, false);
 		list_del(&tmp_list->list);
 		kfree(tmp_list);
 	}
@@ -626,7 +640,14 @@ void cifs_close_deferred_file_under_dentry(struct cifs_tcon *tcon,
 	spin_unlock(&tcon->open_file_lock);
 
 	list_for_each_entry_safe(tmp_list, tmp_next_list, &file_head, list) {
-		_cifsFileInfo_put(tmp_list->cfile, true, false);
+		struct cifsFileInfo *cfile = tmp_list->cfile;
+
+		if (OPEN_FMODE(cfile->f_flags) & FMODE_WRITE) {
+			/* Pairs with smp_load_acquire() in is_size_safe_to_change(). */
+			smp_store_release(&CIFS_I(d_inode(cfile->dentry))->time_last_write,
+					  jiffies);
+		}
+		_cifsFileInfo_put(cfile, true, false);
 		list_del(&tmp_list->list);
 		kfree(tmp_list);
 	}

---
2.55.0