[PATCH 5.10.y] smb: client: fix buffer overflow in SMB2_tcon()

[Sasha Levin <sashal@kernel.org>]

From: Luxiao Xu <rakukuip@gmail.com>

[ Upstream commit 73cc469b0ff7551f8537ea919709973e6dc4f514 ]

In SMB2_tcon(), unc_path is allocated with a fixed buffer size of
MAX_SHARENAME_LENGTH * 2 bytes (672 bytes). However, the user-controlled
UNC path (tree) length is passed to cifs_strtoUTF16(), which assumes the
destination buffer is large enough and writes strlen(tree) + 1 UTF-16 code
units.

Commit 62a1a439e0fd ("cifs: clean up handling of unc= option") removed the
length check on UNC mount options, allowing mount paths to exceed
MAX_SHARENAME_LENGTH while retaining the fixed-size buffer allocation in
SMB2_tcon(). Consequently, an oversized UNC string causes cifs_strtoUTF16()
to write past the end of the heap object, resulting in a slab-out-of-bounds
write. Furthermore, if a tree exceeds MAX_TREE_SIZE characters, strscpy()
truncates it in cifs_tcon::tree_name, causing subsequent reconnects or
failovers to fail.

Fix this by first verifying that the length of tree does not exceed
MAX_TREE_SIZE, returning -EINVAL if it does. Then, dynamically allocate
unc_path based on the length of tree so that it safely accommodates all
UTF-16 code units for paths within MAX_TREE_SIZE.

Fixes: 62a1a439e0fd ("cifs: clean up handling of unc= option")
Cc: stable@vger.kernel.org
Reported-by: Xin Liu <bird@lzu.edu.cn>
Closes: https://bugtracker.nebusec.ai/f/3292
Assisted-by: LLM
Suggested-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
[pc: remove unused MAX_SHARENAME_LENGTH]
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
[ sashal: adjusted context to match the existing MAX_SHARENAME_LENGTH location and direct -EIO return ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/cifs/smb2pdu.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index a2f1d165d8862..bba79784931fe 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -1703,8 +1703,6 @@ static inline void cifs_stats_fail_inc(struct cifs_tcon *tcon, uint16_t code)
 	cifs_stats_inc(&tcon->stats.smb2_stats.smb2_com_failed[code]);
 }
 
-#define MAX_SHARENAME_LENGTH (255 /* server */ + 80 /* share */ + 1 /* NULL */)
-
 /* These are similar values to what Windows uses */
 static inline void init_copy_chunk_defaults(struct cifs_tcon *tcon)
 {
@@ -1738,7 +1736,15 @@ SMB2_tcon(const unsigned int xid, struct cifs_ses *ses, const char *tree,
 	if (!server || !tree)
 		return -EIO;
 
-	unc_path = kmalloc(MAX_SHARENAME_LENGTH * 2, GFP_KERNEL);
+	total_len = strlen(tree);
+	if (total_len > MAX_TREE_SIZE) {
+		cifs_server_dbg(VFS | ONCE,
+				"%s: tree name exceeded %u characters\n",
+				__func__, MAX_TREE_SIZE);
+		return -EINVAL;
+	}
+
+	unc_path = kmalloc((total_len + 1) * sizeof(*unc_path), GFP_KERNEL);
 	if (unc_path == NULL)
 		return -ENOMEM;
 
-- 
2.53.0