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

[Wentao Guan]

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).

Assisted-by: atomcode:glm-5.2
Assisted-by: Codex:gpt-5.5

Fixes: 299f962c0b02 ("ksmbd: use check_add_overflow() to prevent u16 DACL size overflow")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Wentao Guan <guanwentao@xxxxxxxxxxxxx>
---
 fs/smb/server/smbacl.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index 664b1b4a3233d..943fde8300a01 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -644,6 +644,7 @@ 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;
 		}
@@ -658,6 +659,7 @@ 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;
 			}
@@ -703,6 +705,7 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 		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;
 		}
@@ -741,8 +744,10 @@ static void set_ntacl_dacl(struct mnt_idmap *idmap,
 				break;
 
 			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;
+			}
 			aces_size -= nt_ace_size;
 			ntace = (struct smb_ace *)((char *)ntace + nt_ace_size);
 			num_aces++;
-- 
2.30.2