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smb: client: validate dacloffset before building DACL pointers 

parse_sec_desc(), build_sec_desc(), and the chown path in
id_mode_to_cifs_acl() all add the server-supplied dacloffset to pntsd
before proving a DACL header fits inside the returned security
descriptor.

On 32-bit builds a malicious server can return dacloffset near
U32_MAX, wrap the derived DACL pointer below end_of_acl, and then slip
past the later pointer-based bounds checks. build_sec_desc() and
id_mode_to_cifs_acl() can then dereference DACL fields from the wrapped
pointer in the chmod/chown rewrite paths.

Validate dacloffset numerically before building any DACL pointer and
reuse the same helper at the three DACL entry points.

Fixes: bc3e9dd9d1 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
master
Michael Bommarito 2026-04-20 10:47:47 -04:00 committed by Steve French
parent 8d09328dfd
commit f98b48151c
1 changed files with 32 additions and 3 deletions
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35
fs/smb/client/cifsacl.c
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@ -1264,6 +1264,17 @@ static int parse_sid(struct smb_sid *psid, char *end_of_acl)
return 0;
}
static bool dacl_offset_valid(unsigned int acl_len, __u32 dacloffset)
{
if (acl_len < sizeof(struct smb_acl))
return false;
if (dacloffset < sizeof(struct smb_ntsd))
return false;
return dacloffset <= acl_len - sizeof(struct smb_acl);
}
/* Convert CIFS ACL to POSIX form */
static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
@ -1284,7 +1295,6 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
group_sid_ptr = (struct smb_sid *)((char *)pntsd +
le32_to_cpu(pntsd->gsidoffset));
dacloffset = le32_to_cpu(pntsd->dacloffset);
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
cifs_dbg(NOISY, "revision %d type 0x%x ooffset 0x%x goffset 0x%x sacloffset 0x%x dacloffset 0x%x\n",
pntsd->revision, pntsd->type, le32_to_cpu(pntsd->osidoffset),
le32_to_cpu(pntsd->gsidoffset),
@ -1315,11 +1325,18 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
return rc;
}
if (dacloffset)
if (dacloffset) {
if (!dacl_offset_valid(acl_len, dacloffset)) {
cifs_dbg(VFS, "Server returned illegal DACL offset\n");
return -EINVAL;
}
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
parse_dacl(dacl_ptr, end_of_acl, owner_sid_ptr,
group_sid_ptr, fattr, get_mode_from_special_sid);
else
} else {
cifs_dbg(FYI, "no ACL\n"); /* BB grant all or default perms? */
}
return rc;
}
@ -1342,6 +1359,11 @@ static int build_sec_desc(struct smb_ntsd *pntsd, struct smb_ntsd *pnntsd,
dacloffset = le32_to_cpu(pntsd->dacloffset);
if (dacloffset) {
if (!dacl_offset_valid(secdesclen, dacloffset)) {
cifs_dbg(VFS, "Server returned illegal DACL offset\n");
return -EINVAL;
}
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
rc = validate_dacl(dacl_ptr, end_of_acl);
if (rc)
@ -1710,6 +1732,12 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
nsecdesclen = sizeof(struct smb_ntsd) + (sizeof(struct smb_sid) * 2);
dacloffset = le32_to_cpu(pntsd->dacloffset);
if (dacloffset) {
if (!dacl_offset_valid(secdesclen, dacloffset)) {
cifs_dbg(VFS, "Server returned illegal DACL offset\n");
rc = -EINVAL;
goto id_mode_to_cifs_acl_exit;
}
dacl_ptr = (struct smb_acl *)((char *)pntsd + dacloffset);
rc = validate_dacl(dacl_ptr, (char *)pntsd + secdesclen);
if (rc) {
@ -1752,6 +1780,7 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
rc = ops->set_acl(pnntsd, nsecdesclen, inode, path, aclflag);
cifs_dbg(NOISY, "set_cifs_acl rc: %d\n", rc);
}
id_mode_to_cifs_acl_exit:
cifs_put_tlink(tlink);
kfree(pnntsd);