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ksmbd: reject undersized DACLs before parsing ACEs

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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: e2f34481b2 ("cifsd: add server-side procedures for SMB3")
Signed-off-by: Haofeng Li <lihaofeng@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
This commit is contained in:
Haofeng Li 2026-06-25 14:48:07 +00:00 committed by Steve French
parent 7ac657bb9c
commit 60908f7ebc
1 changed files with 6 additions and 1 deletions
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7
fs/smb/server/smbacl.c
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@ -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;