[PATCH v2] smb: client: fix heap overflow in cifs_do_set_acl()

[Frank Sorenson <sorenson@redhat.com>]

cifs_set_acl() validates ACL size using posix_acl_xattr_size():

        4 + (count * 8)  // 4-byte header + 8 bytes per ACE

cifs_do_set_acl() then calls posix_acl_to_cifs() to write the CIFS
wire format into the same buffer:

        6 + (count * 10)  // 6-byte header + 10 bytes per ACE

An ACL that passes the xattr-based check in cifs_set_acl() can
overflow the heap when posix_acl_to_cifs() writes the larger CIFS
format.

Validate the CIFS format size against the remaining buffer space and
USHRT_MAX before converting--data_count is __u16, so sizes above
USHRT_MAX truncate the on-wire packet length, causing the server to
apply a partial ACL.  Replace MaxDataCount = 1000 with
min(CIFSMaxBufSize, USHRT_MAX).

Fixes: dc1af4c4b4721 ("cifs: implement set acl method")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
---
v2 changes:
 - Add USHRT_MAX bound to prevent u16 truncation of data_count for ACLs
   with more than 6553 entries, which would cause a partial ACL to be
   silently applied on the server
 - limit MaxDataCount to min(CIFSMaxBufSize, USHRT_MAX)

 fs/smb/client/cifssmb.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/cifssmb.c b/fs/smb/client/cifssmb.c
index f5aad5f61dce..621aca5d3b75 100644
--- a/fs/smb/client/cifssmb.c
+++ b/fs/smb/client/cifssmb.c
@@ -3555,6 +3555,7 @@ int cifs_do_set_acl(const unsigned int xid, struct cifs_tcon *tcon,
 	int rc = 0;
 	int bytes_returned = 0;
 	__u16 params, byte_count, data_count, param_offset, offset;
+	size_t cifs_acl_size, bytes_available;
 
 	cifs_dbg(FYI, "In SetPosixACL (Unix) for path %s\n", fileName);
 setAclRetry:
@@ -3574,8 +3575,7 @@ int cifs_do_set_acl(const unsigned int xid, struct cifs_tcon *tcon,
 	}
 	params = 6 + name_len;
 	pSMB->MaxParameterCount = cpu_to_le16(2);
-	/* BB find max SMB size from sess */
-	pSMB->MaxDataCount = cpu_to_le16(1000);
+	pSMB->MaxDataCount = cpu_to_le16(min_t(unsigned int, CIFSMaxBufSize, USHRT_MAX));
 	pSMB->MaxSetupCount = 0;
 	pSMB->Reserved = 0;
 	pSMB->Flags = 0;
@@ -3587,6 +3587,15 @@ int cifs_do_set_acl(const unsigned int xid, struct cifs_tcon *tcon,
 	parm_data = ((char *)pSMB) + offset;
 	pSMB->ParameterOffset = cpu_to_le16(param_offset);
 
+	/* make sure we can fit the larger cifs_posix_aces in the buffer */
+	cifs_acl_size = sizeof(struct cifs_posix_acl) +
+		       (acl->a_count * sizeof(struct cifs_posix_ace));
+	bytes_available = (CIFSMaxBufSize + MAX_SMB2_HDR_SIZE) - offset;
+	if (cifs_acl_size > bytes_available || cifs_acl_size > USHRT_MAX) {
+		rc = -E2BIG;
+		goto setACLerrorExit;
+	}
+
 	/* convert to on the wire format for POSIX ACL */
 	data_count = posix_acl_to_cifs(parm_data, acl, acl_type);
 
-- 
2.55.0