From: Hem Parekh Date: Tue, 2 Jun 2026 23:56:46 +0000 (-0700) Subject: ksmbd: fix out-of-bounds read in smb_check_perm_dacl() X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=1ef06004ed4bd6d3ed8c840d9d1a376b66d4935b;p=thirdparty%2Fkernel%2Flinux.git ksmbd: fix out-of-bounds read in smb_check_perm_dacl() The permission-check ACE walk in smb_check_perm_dacl() validates the ACE header size and caps sid.num_subauth at SID_MAX_SUB_AUTHORITIES, but it never checks that ace->size is actually large enough to contain num_subauth sub-authorities before compare_sids() dereferences them. CIFS_SID_BASE_SIZE covers the SID header up to but excluding the sub_auth[] array, and offsetof(struct smb_ace, sid) is the ACE header, so the existing guards only guarantee the 8-byte SID base, i.e. zero sub-authorities. compare_sids() then reads ace->sid.sub_auth[i] for i < min(local_sid->num_subauth, ace->sid.num_subauth). The local comparison SIDs (sid_everyone, sid_unix_NFS_mode, and the id_to_sid() result) always have at least one sub-authority, and an attacker controls the ACE revision and authority bytes (which lie within the in-bounds SID base), so they can match one of those SIDs and force the sub_auth read. A crafted ACE with size == 16 and num_subauth >= 1 placed at the tail of the security descriptor therefore causes a heap out-of-bounds read of up to SID_MAX_SUB_AUTHORITIES * sizeof(__le32) bytes past the pntsd allocation. The security descriptor is loaded by ksmbd_vfs_get_sd_xattr() into a buffer sized exactly to the on-disk data (kzalloc(sd_size) in ndr_decode_v4_ntacl()), so the read lands past the allocation. The malformed descriptor can be stored verbatim via SMB2_SET_INFO (the DACL is not normalised before being written to the security.NTACL xattr) and the read fires on a subsequent SMB2_CREATE access check, making this reachable by an authenticated client on a share that uses ACL xattrs. Add the missing num_subauth-versus-ace_size check, mirroring the identical guards already present in the sibling parsers parse_dacl() and smb_inherit_dacl(). Fixes: d07b26f39246 ("ksmbd: require minimum ACE size in smb_check_perm_dacl()") Cc: stable@vger.kernel.org Signed-off-by: Hem Parekh Acked-by: Namjae Jeon Signed-off-by: Steve French --- diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c index 664b1b4a3233d..340ea98fa494c 100644 --- a/fs/smb/server/smbacl.c +++ b/fs/smb/server/smbacl.c @@ -1477,7 +1477,9 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path, break; aces_size -= ace_size; - if (ace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES) + if (ace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES || + ace_size < offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE + + sizeof(__le32) * ace->sid.num_subauth) break; if (!compare_sids(&sid, &ace->sid) ||