[PATCH] ksmbd: fix named stream write and EOF handling

[PATCH] ksmbd: fix named stream write and EOF handling

[DaeMyung Kang]

An SMB WRITE to an existing named stream can silently discard data after
its written range. ksmbd stores streams as xattr values, but the write
path passes offset + count as the length of the replacement xattr even
when the old stream is longer. Writing 26 bytes at offset 0 to a 60-byte
stream reports 26 bytes written while discarding the remaining 34 bytes.

Keep the existing xattr length when an in-place write ends before EOF.
Reject writes that cross XATTR_SIZE_MAX rather than storing only the
portion that fits while reporting the full requested count.

Commit 4ea0bb8aaedf ("ksmbd: handle set/get info file for streamed file")
intentionally skipped ordinary inode truncation for stream handles: an
EOF SetInfo on an ADS had truncated the base file. That fixed the base
file corruption but left stream EOF changes reporting success without
resizing the stream. Handle these requests by resizing only the stream
xattr; the base file size remains untouched. Zero-fill stream extensions.
This is needed alongside the write fix: a short write's accidental
truncation previously masked an ignored explicit EOF change. The stream
resize helper returns xattr errors directly rather than using the
base-file truncate path's EBADF conversion.

Fixes: f44158485826 ("cifsd: add file operations")
Fixes: 4ea0bb8aaedf ("ksmbd: handle set/get info file for streamed file")
Cc: stable@vger.kernel.org
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
---
Tested with SMB2.1 against QEMU guests running kernels built from
ksmbd-for-next (17a2c1764a45) and cifs-next (ff47652a4b66). On both,
a 26-byte overwrite preserved the tail of a 60-byte stream, while an
explicit EOF=0 followed by the same write left 26 bytes. Shrink,
zero-filled extension, append, middle overwrite and size-limit tests
passed. Both kernels built successfully.

 fs/smb/server/smb2pdu.c |  5 ++--
 fs/smb/server/vfs.c     | 72 +++++++++++++++++++++++++++++++++++++++++++------
 fs/smb/server/vfs.h     |  1 +
 3 files changed, 68 insertions(+), 10 deletions(-)

diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index 26e683aeb..12a43efdb 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -8578,6 +8578,8 @@ static int set_end_of_file_info(struct ksmbd_work *work, struct ksmbd_file *fp,
 
 	newsize = le64_to_cpu(file_eof_info->EndOfFile);
 	inode = file_inode(fp->filp);
+	if (ksmbd_stream_fd(fp))
+		return ksmbd_vfs_stream_truncate(fp, newsize);
 
 	/*
 	 * If FILE_END_OF_FILE_INFORMATION of set_info_file is called
@@ -8586,8 +8588,7 @@ static int set_end_of_file_info(struct ksmbd_work *work, struct ksmbd_file *fp,
 	 * truncate of some filesystem like FAT32 fill zero data in
 	 * truncated range.
 	 */
-	if (inode->i_sb->s_magic != MSDOS_SUPER_MAGIC &&
-	    ksmbd_stream_fd(fp) == false) {
+	if (inode->i_sb->s_magic != MSDOS_SUPER_MAGIC) {
 		ksmbd_debug(SMB, "truncated to newsize %lld\n", newsize);
 		rc = ksmbd_vfs_truncate(work, fp, newsize);
 		if (rc) {
diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
index db0f2de2b..3e0609546 100644
--- a/fs/smb/server/vfs.c
+++ b/fs/smb/server/vfs.c
@@ -388,23 +388,21 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 	const struct cred *saved_cred;
 	char *stream_buf = NULL, *wbuf;
 	struct mnt_idmap *idmap = file_mnt_idmap(fp->filp);
-	size_t size;
+	size_t size, write_end;
 	ssize_t v_len;
 	int err = 0;
 
 	ksmbd_debug(VFS, "write stream data pos : %llu, count : %zd\n",
 		    *pos, count);
 
-	if (*pos >= XATTR_SIZE_MAX) {
+	if (*pos < 0 || *pos >= XATTR_SIZE_MAX) {
 		pr_err("stream write position %lld is out of bounds\n",	*pos);
 		return -EINVAL;
 	}
 
-	size = *pos + count;
-	if (size > XATTR_SIZE_MAX) {
-		size = XATTR_SIZE_MAX;
-		count = XATTR_SIZE_MAX - *pos;
-	}
+	if (count > XATTR_SIZE_MAX - *pos)
+		return -EFBIG;
+	write_end = *pos + count;
 
 	saved_cred = override_creds(fp->filp->f_cred);
 	v_len = ksmbd_vfs_getcasexattr(idmap,
@@ -417,6 +415,8 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 		err = v_len;
 		goto out_revert;
 	}
+	/* Preserve the tail of an existing stream on an in-place write. */
+	size = max_t(size_t, v_len, write_end);
 
 	if (v_len < size) {
 		wbuf = kvzalloc(size, KSMBD_DEFAULT_GFP);
@@ -445,13 +445,69 @@ static int ksmbd_vfs_stream_write(struct ksmbd_file *fp, char *buf, loff_t *pos,
 	if (err < 0)
 		goto out;
 	else
-		fp->stream.pos = size;
+		fp->stream.pos = write_end;
 	err = 0;
 out:
 	kvfree(stream_buf);
 	return err;
 }
 
+/**
+ * ksmbd_vfs_stream_truncate() - change the length of a named stream
+ * @fp:		stream file handle
+ * @newsize:	new stream length
+ *
+ * Resize the stream xattr without changing the base inode size.
+ *
+ * Return: 0 on success, otherwise a negative error code
+ */
+int ksmbd_vfs_stream_truncate(struct ksmbd_file *fp, loff_t newsize)
+{
+	const struct cred *saved_cred;
+	struct mnt_idmap *idmap = file_mnt_idmap(fp->filp);
+	char *stream_buf = NULL, *new_buf = NULL;
+	ssize_t v_len;
+	int err;
+
+	if (newsize < 0)
+		return -EINVAL;
+	if (newsize > XATTR_SIZE_MAX)
+		return -EFBIG;
+
+	saved_cred = override_creds(fp->filp->f_cred);
+	v_len = ksmbd_vfs_getcasexattr(idmap, fp->filp->f_path.dentry,
+				       fp->stream.name, fp->stream.size,
+				       &stream_buf);
+	if (v_len < 0) {
+		err = v_len;
+		goto out;
+	}
+	if (v_len == newsize) {
+		err = 0;
+		goto out;
+	}
+
+	new_buf = stream_buf;
+	if (newsize > v_len) {
+		new_buf = kvzalloc(newsize, KSMBD_DEFAULT_GFP);
+		if (!new_buf) {
+			err = -ENOMEM;
+			goto out;
+		}
+		if (v_len)
+			memcpy(new_buf, stream_buf, v_len);
+	}
+
+	err = ksmbd_vfs_setxattr(idmap, &fp->filp->f_path, fp->stream.name,
+				 new_buf, newsize, 0, true);
+	if (new_buf != stream_buf)
+		kvfree(new_buf);
+out:
+	kvfree(stream_buf);
+	revert_creds(saved_cred);
+	return err;
+}
+
 /**
  * ksmbd_vfs_write() - vfs helper for smb file write
  * @work:	work
diff --git a/fs/smb/server/vfs.h b/fs/smb/server/vfs.h
index 566c670c9..ef3ab3f18 100644
--- a/fs/smb/server/vfs.h
+++ b/fs/smb/server/vfs.h
@@ -83,6 +83,7 @@ int ksmbd_vfs_read(struct ksmbd_work *work, struct ksmbd_file *fp, size_t count,
 int ksmbd_vfs_write(struct ksmbd_work *work, struct ksmbd_file *fp,
 		    char *buf, size_t count, loff_t *pos, bool sync,
 		    ssize_t *written);
+int ksmbd_vfs_stream_truncate(struct ksmbd_file *fp, loff_t newsize);
 int ksmbd_vfs_fsync(struct ksmbd_work *work, u64 fid, u64 p_id);
 int ksmbd_vfs_remove_file(struct ksmbd_work *work, const struct path *path);
 int ksmbd_vfs_link(struct ksmbd_work *work,