[PATCH v2 1/2] ksmbd: skip metadata-only opens in sharing checks

[Ian Bridges <icb@fastmail.org>]

A READ_CONTROL open without sharing can block later reads, writes and
deletes even though it holds no data, execute or delete access.

Keep the existing attribute-only exception. For other pairs, require
both effective access masks to hold one of those rights. MS-FSA
2.1.5.1.2 grants FILE_WRITE_DATA when an open overwrites a file or
stream, or creates a named stream on an existing file. Use that granted
access for sharing checks while keeping the requested access on the
returned handle.

The Time Machine AFP_AfpInfo synthesis keeps its requested access.
ksmbd implements FILE_SUPERSEDE by truncating the file, so count it as
a write.

This also prevents an overwrite from truncating a file while another
open that holds data, execute or delete access does not share writing.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Ian Bridges <icb@fastmail.org>
---

Notes:
    An authenticated user can trigger this on any share, and a guest can on
    a guest-enabled share. Without a stored DACL, directory search
    permission can be enough to hold a metadata-only open without sharing
    and block the file owner's access.
    
    Tested on ksmbd-for-next 957772008502 with KASAN. Targeted tests and
    KUnit matrices covered access masks, sharing modes, create dispositions
    and streams. smbtorture matched the base. There were no KASAN reports or
    warnings.
    
    A reproducer is available on request.
    
    This fix was developed with LLM assistance.

 fs/smb/server/smb2pdu.c    | 30 ++++++++++++++++++++++--------
 fs/smb/server/smb_common.c | 34 +++++++++++++++++++++++++++++++++-
 fs/smb/server/smb_common.h |  5 +++++
 fs/smb/server/vfs_cache.h  |  2 ++
 4 files changed, 62 insertions(+), 9 deletions(-)

diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index b084cca8813c..5bbc33d5a3a8 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -3541,7 +3541,8 @@ static int smb2_set_ea(struct smb2_ea_info *eabuf, unsigned int buf_len,
 
 static noinline int smb2_set_stream_name_xattr(const struct path *path,
 					       struct ksmbd_file *fp,
-					       char *stream_name, int s_type)
+					       char *stream_name, int s_type,
+					       bool *created)
 {
 	const struct mnt_idmap *idmap = mnt_idmap(path->mnt);
 	size_t xattr_stream_size;
@@ -3605,6 +3606,8 @@ static noinline int smb2_set_stream_name_xattr(const struct path *path,
 	rc = ksmbd_vfs_setxattr(idmap, path, xattr_stream_name, NULL, 0, 0, false);
 	if (rc < 0)
 		pr_err("Failed to store XATTR stream name :%d\n", rc);
+	else
+		*created = true;
 	return 0;
 }
 
@@ -4394,6 +4397,7 @@ int smb2_open(struct ksmbd_work *work)
 	char *stream_name = NULL;
 	bool file_present = false, created = false, already_permitted = false;
 	bool app_instance_id_checked = false, app_instance_closed;
+	bool stream_created = false;
 	int share_ret, need_truncate = 0;
 	u64 time, alloc_size = 0;
 	umode_t posix_mode = 0;
@@ -5138,7 +5142,8 @@ int smb2_open(struct ksmbd_work *work)
 		rc = smb2_set_stream_name_xattr(&path,
 						fp,
 						stream_name,
-						s_type);
+						s_type,
+						&stream_created);
 		if (rc)
 			goto err_out;
 		file_info = FILE_CREATED;
@@ -5146,13 +5151,25 @@ int smb2_open(struct ksmbd_work *work)
 
 	fp->attrib_only = !(req->DesiredAccess & ~(FILE_READ_ATTRIBUTES_LE |
 			FILE_WRITE_ATTRIBUTES_LE | FILE_SYNCHRONIZE_LE));
+	need_truncate = !S_ISDIR(file_inode(filp)->i_mode) &&
+			open_flags & O_TRUNC && !fp->attrib_only && !stream_name;
+	/*
+	 * Count the FILE_WRITE_DATA that MS-FSA 2.1.5.1.2 grants an overwrite,
+	 * supersede included, or a new named stream on an existing file.
+	 * An attribute-only open of the file is not truncated, and the
+	 * synthesized AFP_AfpInfo stream is not created by the client.
+	 */
+	fp->implied_write = need_truncate ||
+			    (stream_name &&
+			     (open_flags & O_TRUNC ||
+			      (file_present && stream_created)));
 
 	fp->is_posix_ctxt = posix_ctxt;
 	fp->posix_append = posix_ctxt && !!(open_flags & O_APPEND);
 
 	/* fp should be searchable through ksmbd_inode.m_fp_list
-	 * after daccess, saccess, attrib_only, and stream are
-	 * initialized.
+	 * after daccess, saccess, attrib_only, implied_write, and
+	 * stream are initialized.
 	 */
 	down_write(&fp->f_ci->m_lock);
 	list_add(&fp->node, &fp->f_ci->m_fp_list);
@@ -5173,11 +5190,8 @@ int smb2_open(struct ksmbd_work *work)
 	if (file_present || created)
 		path_put(&path);
 
-	if (!S_ISDIR(file_inode(filp)->i_mode) && open_flags & O_TRUNC &&
-	    !fp->attrib_only && !stream_name) {
+	if (need_truncate)
 		smb_break_all_oplock(work, fp);
-		need_truncate = 1;
-	}
 
 	share_ret = ksmbd_smb_check_shared_mode(fp->filp, fp);
 	if (!test_share_config_flag(work->tcon->share_conf, KSMBD_SHARE_FLAG_OPLOCKS) ||
diff --git a/fs/smb/server/smb_common.c b/fs/smb/server/smb_common.c
index f9fb3741ada3..e7a987af64ba 100644
--- a/fs/smb/server/smb_common.c
+++ b/fs/smb/server/smb_common.c
@@ -695,10 +695,24 @@ static void smb_shared_mode_error(int error, struct ksmbd_file *prev_fp,
 		    prev_fp->saccess, curr_fp->daccess);
 }
 
+/*
+ * Sharing checks use fp->daccess plus the FILE_WRITE_DATA that MS-FSA
+ * grants an open that overwrites data or creates a named stream.
+ * fp->daccess keeps the open's own access.
+ */
+__le32 ksmbd_sharing_daccess(const struct ksmbd_file *fp)
+{
+	if (fp->implied_write)
+		return fp->daccess | FILE_WRITE_DATA_LE;
+	return fp->daccess;
+}
+
 int ksmbd_smb_check_shared_mode(struct file *filp, struct ksmbd_file *curr_fp)
 {
 	int rc = 0;
 	struct ksmbd_file *prev_fp;
+	__le32 curr_access = ksmbd_sharing_daccess(curr_fp);
+	__le32 access;
 
 	/*
 	 * Lookup fp in master fp list, and check desired access and
@@ -719,6 +733,24 @@ int ksmbd_smb_check_shared_mode(struct file *filp, struct ksmbd_file *curr_fp)
 		if (prev_fp->attrib_only != curr_fp->attrib_only)
 			continue;
 
+		/*
+		 * MS-FSA checks sharing per stream, so a named stream open's
+		 * implied write is not a write against the primary stream.
+		 * It still decides whether the pair is checked.
+		 */
+		access = curr_access;
+		if (ksmbd_stream_fd(curr_fp) && !ksmbd_stream_fd(prev_fp))
+			access = curr_fp->daccess;
+
+		/*
+		 * Two opens conflict only if both hold a right in
+		 * FILE_SHARE_CHECKED_ACCESS_LE (MS-FSA 2.1.5.1.2.1 and
+		 * 2.1.5.1.2.2)
+		 */
+		if (!(ksmbd_sharing_daccess(prev_fp) & FILE_SHARE_CHECKED_ACCESS_LE) ||
+		    !(curr_access & FILE_SHARE_CHECKED_ACCESS_LE))
+			continue;
+
 		if (!(prev_fp->saccess & FILE_SHARE_DELETE_LE) &&
 		    curr_fp->daccess & FILE_DELETE_LE) {
 			smb_shared_mode_error(SHARE_DELETE_ERROR,
@@ -745,7 +777,7 @@ int ksmbd_smb_check_shared_mode(struct file *filp, struct ksmbd_file *curr_fp)
 		}
 
 		if (!(prev_fp->saccess & FILE_SHARE_WRITE_LE) &&
-		    curr_fp->daccess & (FILE_WRITE_DATA_LE | FILE_APPEND_DATA_LE)) {
+		    access & (FILE_WRITE_DATA_LE | FILE_APPEND_DATA_LE)) {
 			smb_shared_mode_error(SHARE_WRITE_ERROR,
 					      prev_fp,
 					      curr_fp);
diff --git a/fs/smb/server/smb_common.h b/fs/smb/server/smb_common.h
index a9cc09e31721..d3d19f1a23ea 100644
--- a/fs/smb/server/smb_common.h
+++ b/fs/smb/server/smb_common.h
@@ -69,6 +69,10 @@
 		FILE_EXECUTE | FILE_DELETE_CHILD | \
 		FILE_READ_ATTRIBUTES | FILE_WRITE_ATTRIBUTES)
 
+/* Rights that make an open take part in sharing checks */
+#define FILE_SHARE_CHECKED_ACCESS_LE	(FILE_EXECUTE_LE | FILE_READ_DATA_LE | \
+		FILE_WRITE_DATA_LE | FILE_APPEND_DATA_LE | FILE_DELETE_LE)
+
 #define SMB_COM_NEGOTIATE		0x72 /* See MS-CIFS 2.2.2.1 */
 
 /* See MS-CIFS 2.2.3.1 */
@@ -284,6 +288,7 @@ int ksmbd_extract_shortname(struct ksmbd_conn *conn,
 
 int ksmbd_smb_negotiate_common(struct ksmbd_work *work, unsigned int command);
 
+__le32 ksmbd_sharing_daccess(const struct ksmbd_file *fp);
 int ksmbd_smb_check_shared_mode(struct file *filp, struct ksmbd_file *curr_fp);
 int __ksmbd_override_fsids(struct ksmbd_work *work,
 			   struct ksmbd_share_config *share);
diff --git a/fs/smb/server/vfs_cache.h b/fs/smb/server/vfs_cache.h
index a0239ec178d8..57c81287efe6 100644
--- a/fs/smb/server/vfs_cache.h
+++ b/fs/smb/server/vfs_cache.h
@@ -131,6 +131,8 @@ struct ksmbd_file {
 
 	bool				is_nt_open;
 	bool				attrib_only;
+	/* writes data on open, see ksmbd_sharing_daccess() */
+	bool				implied_write;
 	bool				allocation_size_set;
 
 	char				client_guid[16];
-- 
2.47.3