From 16fb65ec15fe7c90f50a2115854bfd9a032d4023 Mon Sep 17 00:00:00 2001 From: Haofeng Li Date: Thu, 25 Jun 2026 14:48:07 +0000 Subject: [PATCH] ksmbd: reject undersized DACLs before parsing ACEs [ Upstream commit 60908f7ebcd9b6cde74ad5711fab0f49c7970949 ] parse_dacl() limits the attacker-controlled ACE count by comparing it with the number of minimal ACEs that fit in the DACL size. The DACL size field is 16 bits, but the expression subtracts sizeof(struct smb_acl). Because sizeof() is unsigned, a DACL size smaller than the ACL header underflows to a large size_t. A malicious client can reach this with: SMB2_SET_INFO (InfoType=SMB2_O_INFO_SECURITY) -> smb2_set_info_sec() -> set_info_sec() -> parse_sec_desc() -> parse_dacl() -> init_acl_state(..., 0xffff) -> init_acl_state(..., 0xffff) -> kmalloc_objs(..., 0xffff) Thus a malformed security descriptor can make num_aces pass the guard and drive large temporary ACL state and pointer-array allocations. Reject DACLs smaller than struct smb_acl before doing the subtraction, so the ACE count check cannot be bypassed by the underflow. Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3") Signed-off-by: Haofeng Li Reviewed-by: ChenXiaoSong Acked-by: Namjae Jeon Signed-off-by: Steve French Signed-off-by: Sasha Levin --- 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 cd6c72fa1960..420d4e0733e5 100644 --- a/fs/smb/server/smbacl.c +++ b/fs/smb/server/smbacl.c @@ -374,6 +374,7 @@ static void parse_dacl(struct mnt_idmap *idmap, { int i, ret; u16 num_aces = 0; + u16 dacl_size; unsigned int acl_size; char *acl_base; struct smb_ace **ppace; @@ -403,7 +404,11 @@ static void parse_dacl(struct mnt_idmap *idmap, if (num_aces <= 0) return; - if (num_aces > (le16_to_cpu(pdacl->size) - sizeof(struct smb_acl)) / + dacl_size = le16_to_cpu(pdacl->size); + if (dacl_size < sizeof(struct smb_acl)) + return; + + if (num_aces > (dacl_size - sizeof(struct smb_acl)) / (offsetof(struct smb_ace, sid) + offsetof(struct smb_sid, sub_auth) + sizeof(__le16))) return;