[PATCH v2 4/5] smb: client: avoid using uninitialized SIDs in cifs_posix_to_fattr()

[Paulo Alcantara <pc@manguebit.org>]

cifs_posix_to_fattr() ignores the return value of posix_info_parse().
When a malformed POSIX directory entry is encountered (e.g. invalid
SID lengths from an untrusted server), posix_info_parse() returns -1
without populating the 'parsed' struct.  The uninitialized stack
memory in parsed.owner and parsed.group is then passed to
sid_to_id(), which processes the garbage bytes and passes them to
request_key() to construct a SID string, potentially leaking kernel
stack contents to the userspace idmap daemon.

Fix this by checking the return value and skipping the SID-to-id
mapping when parsing fails.  The remaining fattr fields (timestamps,
mode, etc.) are populated directly from the 'info' pointer so they
are unaffected.

Closes: https://sashiko.dev/#/patchset/20260906172005.627163-1-pc%40manguebit.org
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
---
 fs/smb/client/readdir.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/smb/client/readdir.c b/fs/smb/client/readdir.c
index 1ea84f4ada39..44c5c863bd2d 100644
--- a/fs/smb/client/readdir.c
+++ b/fs/smb/client/readdir.c
@@ -244,8 +244,9 @@ cifs_posix_to_fattr(struct cifs_fattr *fattr, struct smb2_posix_info *info,
 {
 	unsigned int sbflags = cifs_sb_flags(cifs_sb);
 	struct smb2_posix_info_parsed parsed;
+	int rc;
 
-	posix_info_parse(info, NULL, &parsed);
+	rc = posix_info_parse(info, NULL, &parsed);
 
 	memset(fattr, 0, sizeof(*fattr));
 	fattr->cf_uniqueid = le64_to_cpu(info->Inode);
@@ -284,10 +285,15 @@ cifs_posix_to_fattr(struct cifs_fattr *fattr, struct smb2_posix_info *info,
 
 	fattr->cf_uid = cifs_sb->ctx->linux_uid;
 	fattr->cf_gid = cifs_sb->ctx->linux_gid;
-	if (!(sbflags & CIFS_MOUNT_OVERR_UID))
-		sid_to_id(cifs_sb, &parsed.owner, fattr, SIDOWNER);
-	if (!(sbflags & CIFS_MOUNT_OVERR_GID))
-		sid_to_id(cifs_sb, &parsed.group, fattr, SIDGROUP);
+	if (rc) {
+		cifs_dbg(VFS, "%s: failed to parse SIDs: %d\n",
+			 __func__, rc);
+	} else {
+		if (!(sbflags & CIFS_MOUNT_OVERR_UID))
+			sid_to_id(cifs_sb, &parsed.owner, fattr, SIDOWNER);
+		if (!(sbflags & CIFS_MOUNT_OVERR_GID))
+			sid_to_id(cifs_sb, &parsed.group, fattr, SIDGROUP);
+	}
 }
 
 static void __dir_info_to_fattr(struct cifs_fattr *fattr, const void *info)
-- 
2.55.0