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

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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: 299f962c0b ("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>
This commit is contained in:
Wentao Guan 2026-07-03 11:22:09 +09:00 committed by Steve French
parent 58d97fcd0b
commit bbf0a8e931
1 changed files with 6 additions and 1 deletions
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7
fs/smb/server/smbacl.c
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@ -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 @@ 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;
}
@ -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: