{"id":2231472,"url":"http://patchwork.ozlabs.org/api/patches/2231472/?format=json","web_url":"http://patchwork.ozlabs.org/project/linux-cifs-client/patch/20260430-kerbmi-v2-2-0b98fe250425@microsoft.com/","project":{"id":12,"url":"http://patchwork.ozlabs.org/api/projects/12/?format=json","name":"Linux CIFS Client","link_name":"linux-cifs-client","list_id":"linux-cifs.vger.kernel.org","list_email":"linux-cifs@vger.kernel.org","web_url":"","scm_url":"","webscm_url":"","list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"<20260430-kerbmi-v2-2-0b98fe250425@microsoft.com>","list_archive_url":null,"date":"2026-04-30T17:48:24","name":"[v2,2/2] smb: client: Zero-pad short GSS session keys per MS-SMB2","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"a36825cdbc0705dd5a16e8a3591664d3bf04133b","submitter":{"id":92318,"url":"http://patchwork.ozlabs.org/api/people/92318/?format=json","name":"Piyush Sachdeva","email":"s.piyush1024@gmail.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/linux-cifs-client/patch/20260430-kerbmi-v2-2-0b98fe250425@microsoft.com/mbox/","series":[{"id":502352,"url":"http://patchwork.ozlabs.org/api/series/502352/?format=json","web_url":"http://patchwork.ozlabs.org/project/linux-cifs-client/list/?series=502352","date":"2026-04-30T17:48:22","name":"smb: client: Spec-compliance fixes for Kerberos key derivation","version":2,"mbox":"http://patchwork.ozlabs.org/series/502352/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2231472/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2231472/checks/","tags":{},"related":[],"headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-cifs@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=ajX4S1oZ;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c0a:e001:db::12fc:5321; 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charset=\"utf-8\"","Content-Transfer-Encoding":"7bit","Message-Id":"<20260430-kerbmi-v2-2-0b98fe250425@microsoft.com>","References":"<20260430-kerbmi-v2-0-0b98fe250425@microsoft.com>","In-Reply-To":"<20260430-kerbmi-v2-0-0b98fe250425@microsoft.com>","To":"Steve French , linux-cifs@vger.kernel.org,\n Shyam Prasad N ,\n Bharath SM ","Cc":"samba-technical@lists.samba.org, linux-kernel@vger.kernel.org,\n vaibsharma@microsoft.com","X-Mailer":"b4 0.15.2","X-Developer-Signature":"v=1; a=openpgp-sha256; l=3414;\n i=psachdeva@microsoft.com; h=from:subject:message-id;\n bh=aHoBqTbdr56DO71Zlzzaandq5SHQ8sCJYyYln2KmSW0=;\n b=owGbwMvMwCV29FJ3ncRHDT/G02pJDJmfp36Y1SsyOXPuWc+60JDNV/zit7Tq/pv3nulpyY+Li\n Y/fX9es7JjIwiDGxWAppsiy4cQdWd74XZLzPj0xgpnDygQyRFqkgQEIWBj4chPzSo10jPRMtQ31\n DI10DHSMGbg4BWCqk70YGS7MWcO/smkdz+xDRxMlFA+wFglfelvZMl18p1FeW8XMRm5Ghs9LPzf\n KHc2z6mDWbLqhnLiEwzTQV+9R897QyJ1aWy8z8AMA","X-Developer-Key":"i=psachdeva@microsoft.com; a=openpgp;\n fpr=80350F71F916134953C3EB979E19C6F9839C3CFC"},"content":"Per MS-SMB2 section 3.2.5.3, Session.SessionKey is the first 16 bytes\nof the GSS cryptographic key, right-padded with zero bytes if the key\nis shorter than 16 bytes.\n\nSMB2_auth_kerberos() copies the GSS session key from the cifs.upcall\nresponse using kmemdup(msg->data, msg->sesskey_len, ...) and stores\nthe GSS-reported length verbatim in ses->auth_key.len. generate_key()\nreads SMB2_NTLMV2_SESSKEY_SIZE bytes from this buffer when feeding the\nHMAC-SHA256 KDF for signing key derivation. If a GSS mechanism returns\na session key shorter than 16 bytes (e.g. a deprecated single-DES\nKerberos enctype with an 8-byte session key), the KDF call performs an\nout-of-bounds slab read and derives keys that do not match the server,\nwhich pads per the spec.\n\nModern KDCs disable short-key enctypes by default, so this is latent\nrather than reachable in production, but it is still a kernel heap\nover-read.\n\nAllocate auth_key.response with kzalloc() at a length of\nmax(msg->sesskey_len, SMB2_NTLMV2_SESSKEY_SIZE), copy the GSS key in,\nand rely on kzalloc()'s zero initialization for the spec-mandated\npadding. Set ses->auth_key.len to the padded length. Larger GSS keys\n(e.g. the 32-byte aes256-cts-hmac-sha1-96 session key) continue to be\nstored at their natural length, preserving the FullSessionKey path.\n\nEmit a cifs_dbg(VFS, ...) message when a short key is encountered to\nsurface deprecated-enctype usage.\n\nNTLMv2 and NTLMSSP code paths produce a 16-byte session key by\nconstruction and are unaffected.\n\nSigned-off-by: Piyush Sachdeva \nSigned-off-by: Piyush Sachdeva \n---\n fs/smb/client/smb2pdu.c | 23 ++++++++++++++++++-----\n 1 file changed, 18 insertions(+), 5 deletions(-)","diff":"diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c\nindex cb61051f9af3..995fcdd30681 100644\n--- a/fs/smb/client/smb2pdu.c\n+++ b/fs/smb/client/smb2pdu.c\n@@ -1713,17 +1713,30 @@ SMB2_auth_kerberos(struct SMB2_sess_data *sess_data)\n \tis_binding = (ses->ses_status == SES_GOOD);\n \tspin_unlock(&ses->ses_lock);\n \n+\t/*\n+\t * Per MS-SMB2 3.2.5.3, Session.SessionKey is the first 16 bytes of the\n+\t * GSS cryptographic key, right-padded with zero bytes if shorter.\n+\t * Allocate at least SMB2_NTLMV2_SESSKEY_SIZE bytes (zeroed) so the KDF\n+\t * input buffer is always valid for HMAC-SHA256 even with deprecated\n+\t * Kerberos enctypes that return a short session key.\n+\t */\n+\tif (unlikely(msg->sesskey_len < SMB2_NTLMV2_SESSKEY_SIZE))\n+\t\tcifs_dbg(VFS,\n+\t\t\t \"short GSS session key (%u bytes); zero-padding per MS-SMB2 3.2.5.3\\n\",\n+\t\t\t msg->sesskey_len);\n+\n \tkfree_sensitive(ses->auth_key.response);\n-\tses->auth_key.response = kmemdup(msg->data,\n-\t\t\t\t\t msg->sesskey_len,\n-\t\t\t\t\t GFP_KERNEL);\n+\tses->auth_key.len = max_t(unsigned int, msg->sesskey_len,\n+\t\t\t\t SMB2_NTLMV2_SESSKEY_SIZE);\n+\tses->auth_key.response = kzalloc(ses->auth_key.len, GFP_KERNEL);\n \tif (!ses->auth_key.response) {\n \t\tcifs_dbg(VFS, \"%s: can't allocate (%u bytes) memory\\n\",\n-\t\t\t __func__, msg->sesskey_len);\n+\t\t\t __func__, ses->auth_key.len);\n+\t\tses->auth_key.len = 0;\n \t\trc = -ENOMEM;\n \t\tgoto out_put_spnego_key;\n \t}\n-\tses->auth_key.len = msg->sesskey_len;\n+\tmemcpy(ses->auth_key.response, msg->data, msg->sesskey_len);\n \n \tsess_data->iov[1].iov_base = msg->data + msg->sesskey_len;\n \tsess_data->iov[1].iov_len = msg->secblob_len;\n","prefixes":["v2","2/2"]}