ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL

check_add_overflow() unconditionally writes the truncated sum into *d
even on overflow, per its contract in include/linux/overflow.h.
The four check_add_overflow() guards in set_posix_acl_entries_dacl()
and set_ntacl_dacl() break out of the ACE-building loops on overflow,
but the truncated *size is then consumed downstream at the end of
set_ntacl_dacl():

    pndacl->size = cpu_to_le16(le16_to_cpu(pndacl->size) + size);

This produces an on-wire NT ACL whose pndacl->size under-reports the
bytes actually written by the preceding fill_ace_for_sid()/memcpy()
calls, yielding a malformed ACL that can trigger out-of-bounds reads
when re-parsed by clients or ksmbd itself.

Restore *size to its pre-addition value on each overflow branch (via
`*size -= ace_sz` / `size -= nt_ace_size`) so that after the break,
*size once again holds the cumulative size of the successfully-written
ACEs. The committed ACL is then truncated-but-self-consistent rather
than malformed.

The ksmbd DACL builders are the only check_add_overflow() sites found
where an overflow path breaks out of a loop and the destination value
is consumed afterward. The other nearby break-style cases either
return -EINVAL on overflow (transport_ipc.c) or break without
consuming the overflowed destination value afterward (buildid.c).

Fixes: 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16 DACL size overflow")
Assisted-by: atomcode:glm-5.2
Assisted-by: Codex:gpt-5.5
Cc: stable@vger.kernel.org
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index d28289188e44952415b7208e0466591f06050b67..f285b4f24a5bd2a988584a7dfe4d99b00799d515 100644 (file)
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -663,6 +663,7 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
                ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, flags,
                                pace->e_perm, 0777);
                if (check_add_overflow(*size, ace_sz, size)) {
+                       *size -= ace_sz;
                        kfree(sid);
                        break;
                }
@@ -677,6 +678,7 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
                        ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED,
                                        0x03, pace->e_perm, 0777);
                        if (check_add_overflow(*size, ace_sz, size)) {
+                               *size -= ace_sz;
                                kfree(sid);
                                break;
                        }
@@ -722,6 +724,7 @@ posix_default_acl:
                ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, 0x0b,
                                pace->e_perm, 0777);
                if (check_add_overflow(*size, ace_sz, size)) {
+                       *size -= ace_sz;
                        kfree(sid);
                        break;
                }
@@ -765,8 +768,10 @@ static void set_ntacl_dacl(struct mnt_idmap *idmap,
                                goto next_ace;
 
                        memcpy((char *)pndace + size, ntace, nt_ace_size);
-                       if (check_add_overflow(size, nt_ace_size, &size))
+                       if (check_add_overflow(size, nt_ace_size, &size)) {
+                               size -= nt_ace_size;
                                break;
+                       }
                        num_aces++;
 
 next_ace: