Hi Steve,

Please accept this fix for the bug that Colin pointed out.
This can be hit if the server has a corrupted SD, or it got corrupted
over the network.
We used to ignore the ACL in such a case (which in combination with my
patches caused the issue). But I think we should be returning an error
immediately.

Regards,
Shyam

On Wed, Feb 24, 2021 at 7:16 AM Shyam Prasad <Shyam.Prasad@microsoft.com> wrote:
>
> Hi Colin,
>
> Thanks for reporting this. I'll submit a fix.
>
> Regards,
> Shyam
>
> -----Original Message-----
> From: Colin Ian King <colin.king@canonical.com>
> Sent: Wednesday, February 24, 2021 6:14 PM
> To: Shyam Prasad <Shyam.Prasad@microsoft.com>
> Cc: Steve French <sfrench@samba.org>; linux-cifs@vger.kernel.org; samba-technical@lists.samba.org; linux-kernel@vger.kernel.org
> Subject: [EXTERNAL] re: cifs: Retain old ACEs when converting between mode bits and ACL.
>
> Hi,
>
> Static analysis on linux-next with Coverity had detected a potential null pointer dereference with the following commit:
>
> commit f5065508897a922327f32223082325d10b069ebc
> Author: Shyam Prasad N <sprasad@microsoft.com>
> Date:   Fri Feb 12 04:38:43 2021 -0800
>
>     cifs: Retain old ACEs when converting between mode bits and ACL.
>
> The analysis is as follows:
>
> 1258 /* Convert permission bits from mode to equivalent CIFS ACL */
> 1259 static int build_sec_desc(struct cifs_ntsd *pntsd, struct cifs_ntsd *pnntsd,
> 1260        __u32 secdesclen, __u32 *pnsecdesclen, __u64 *pnmode, kuid_t
> uid, kgid_t gid,
> 1261        bool mode_from_sid, bool id_from_sid, int *aclflag)
> 1262 {
> 1263        int rc = 0;
> 1264        __u32 dacloffset;
> 1265        __u32 ndacloffset;
> 1266        __u32 sidsoffset;
> 1267        struct cifs_sid *owner_sid_ptr, *group_sid_ptr;
> 1268        struct cifs_sid *nowner_sid_ptr = NULL, *ngroup_sid_ptr = NULL;
>
>     1. assign_zero: Assigning: dacl_ptr = NULL.
>
> 1269        struct cifs_acl *dacl_ptr = NULL;  /* no need for SACL ptr */
> 1270        struct cifs_acl *ndacl_ptr = NULL; /* no need for SACL ptr */
> 1271        char *end_of_acl = ((char *)pntsd) + secdesclen;
> 1272        u16 size = 0;
> 1273
> 1274        dacloffset = le32_to_cpu(pntsd->dacloffset);
>
>     2. Condition dacloffset, taking false branch.
>
> 1275        if (dacloffset) {
> 1276                dacl_ptr = (struct cifs_acl *)((char *)pntsd +
> dacloffset);
> 1277                if (end_of_acl < (char *)dacl_ptr +
> le16_to_cpu(dacl_ptr->size)) {
> 1278                        cifs_dbg(VFS, "Existing ACL size is wrong.
> Discarding old ACL\n");
> 1279                        dacl_ptr = NULL;
>
> NOTE: dacl_ptr is set to NULL and dacloffset is true
>
> 1280                }
> 1281        }
> 1282
> 1283        owner_sid_ptr = (struct cifs_sid *)((char *)pntsd +
> 1284                        le32_to_cpu(pntsd->osidoffset));
> 1285        group_sid_ptr = (struct cifs_sid *)((char *)pntsd +
> 1286                        le32_to_cpu(pntsd->gsidoffset));
> 1287
>
>     3. Condition pnmode, taking true branch.
>     4. Condition *pnmode != 18446744073709551615ULL, taking false branch.
>
> 1288        if (pnmode && *pnmode != NO_CHANGE_64) { /* chmod */
> 1289                ndacloffset = sizeof(struct cifs_ntsd);
> 1290                ndacl_ptr = (struct cifs_acl *)((char *)pnntsd +
> ndacloffset);
> 1291                ndacl_ptr->revision =
> 1292                        dacloffset ? dacl_ptr->revision :
> cpu_to_le16(ACL_REVISION);
> 1293
> 1294                ndacl_ptr->size = cpu_to_le16(0);
> 1295                ndacl_ptr->num_aces = cpu_to_le32(0);
> 1296
> 1297                rc = set_chmod_dacl(dacl_ptr, ndacl_ptr,
> owner_sid_ptr, group_sid_ptr,
> 1298                                    pnmode, mode_from_sid);
> 1299
> 1300                sidsoffset = ndacloffset + le16_to_cpu(ndacl_ptr->size);
> 1301                /* copy the non-dacl portion of secdesc */
> 1302                *pnsecdesclen = copy_sec_desc(pntsd, pnntsd, sidsoffset,
> 1303                                NULL, NULL);
> 1304
> 1305                *aclflag |= CIFS_ACL_DACL;
> 1306        } else {
> 1307                ndacloffset = sizeof(struct cifs_ntsd);
> 1308                ndacl_ptr = (struct cifs_acl *)((char *)pnntsd +
> ndacloffset);
>
>     5. Condition dacloffset, taking false branch.
>
> 1309                ndacl_ptr->revision =
> 1310                        dacloffset ? dacl_ptr->revision :
> cpu_to_le16(ACL_REVISION);
>
>     Explicit null dereferenced (FORWARD_NULL)
>
>     6. var_deref_op: Dereferencing null pointer dacl_ptr.
>
> 1311                ndacl_ptr->num_aces = dacl_ptr->num_aces;
>
>
> Line 1309..1311, when dacloffset and dacl_ptr is null we hit a null ptr dereference on dacl_ptr.
>


-- 
Regards,
Shyam