[PATCH v3] smb: client: restrict implied bcc[0] exemption to responses without data area

smb2_check_message() has a long-standing quirk that accepts a response
whose calculated length is one byte larger than the bytes actually
received ("server can return one byte more due to implied bcc[0]").
This was introduced to accommodate servers that omit the trailing bcc[0]
overlap byte when no data area is present.

However, the exemption is applied unconditionally, regardless of whether
the command actually carries a data area (has_smb2_data_area[]).  When a
response with a data area is subject to the +1 exemption, the reported
data can extend one byte beyond the bytes actually received, yet
smb2_check_message() still accepts it.  The subsequent decoder then reads
past the end of the receive buffer.  This is reachable during NEGOTIATE
and SESSION_SETUP, before the session is established.

The resulting out-of-bounds reads are visible under KASAN when mounting
against a non-conforming server; both the SPNEGO/negTokenInit and the
NTLMSSP challenge decoders are affected:

  BUG: KASAN: slab-out-of-bounds in asn1_ber_decoder+0x16a7/0x1b00
  Read of size 1 at addr ffff8880084d67c0 by task mount.cifs/81
  CPU: 1 UID: 0 PID: 81 Comm: mount.cifs Not tainted 7.1.0-rc6 #1
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4e/0x70
   print_report+0x157/0x4c9
   kasan_report+0xce/0x100
   asn1_ber_decoder+0x16a7/0x1b00
   decode_negTokenInit+0x19/0x30
   SMB2_negotiate+0x31d9/0x4c90
   cifs_negotiate_protocol+0x1f2/0x3f0
   cifs_get_smb_ses+0x93f/0x17e0
   cifs_mount_get_session+0x7f/0x3a0
   cifs_mount+0xb4/0xcf0
   cifs_smb3_do_mount+0x23a/0x1500
   smb3_get_tree+0x3b0/0x630
   vfs_get_tree+0x82/0x2d0
   fc_mount+0x10/0x1b0
   path_mount+0x50d/0x1de0
   __x64_sys_mount+0x20b/0x270
   do_syscall_64+0xee/0x590
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  Allocated by task 85:
   kmem_cache_alloc_noprof+0x106/0x380
   mempool_alloc_noprof+0x116/0x1e0
   cifs_small_buf_get+0x31/0x80
   allocate_buffers+0x10d/0x2b0
   cifs_demultiplex_thread+0x1d5/0x1d50
   kthread+0x2c6/0x390
   ret_from_fork+0x36e/0x5a0
   ret_from_fork_asm+0x1a/0x30
  The buggy address is located 0 bytes to the right of
   allocated 448-byte region [ffff8880084d6600, ffff8880084d67c0)
   which belongs to the cache cifs_small_rq of size 448

  BUG: KASAN: slab-out-of-bounds in kmemdup_noprof+0x36/0x50
  Read of size 329 at addr ffff88800726c678 by task mount.cifs/89
  CPU: 0 UID: 0 PID: 89 Comm: mount.cifs Tainted: G    B      7.1.0-rc6 #1
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4e/0x70
   print_report+0x157/0x4c9
   kasan_report+0xce/0x100
   kasan_check_range+0x10f/0x1e0
   __asan_memcpy+0x23/0x60
   kmemdup_noprof+0x36/0x50
   decode_ntlmssp_challenge+0x457/0x680
   SMB2_sess_auth_rawntlmssp_negotiate+0x6f0/0xcb0
   SMB2_sess_setup+0x219/0x4f0
   cifs_setup_session+0x248/0xaf0
   cifs_get_smb_ses+0xf79/0x17e0
   cifs_mount_get_session+0x7f/0x3a0
   cifs_mount+0xb4/0xcf0
   cifs_smb3_do_mount+0x23a/0x1500
   smb3_get_tree+0x3b0/0x630
   vfs_get_tree+0x82/0x2d0
   fc_mount+0x10/0x1b0
   path_mount+0x50d/0x1de0
   __x64_sys_mount+0x20b/0x270
   do_syscall_64+0xee/0x590
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  Allocated by task 93:
   kmem_cache_alloc_noprof+0x106/0x380
   mempool_alloc_noprof+0x116/0x1e0
   cifs_small_buf_get+0x31/0x80
   allocate_buffers+0x10d/0x2b0
   cifs_demultiplex_thread+0x1d5/0x1d50
   kthread+0x2c6/0x390
   ret_from_fork+0x36e/0x5a0
   ret_from_fork_asm+0x1a/0x30
  The buggy address is located 120 bytes inside of
   allocated 448-byte region [ffff88800726c600, ffff88800726c7c0)
   which belongs to the cache cifs_small_rq of size 448

Restrict the +1 exemption to responses that have no data area, so that
it still covers the bcc[0] omission it was meant for.  When a data area
is present, the +1 discrepancy instead means the reported data length
overruns the received buffer, so the response must be rejected.

Fixes: 093b2bdad322 ("CIFS: Make demultiplex_thread work with SMB2 code")
Cc: [email protected]
Signed-off-by: Shoichiro Miyamoto <[email protected]>
---
v3: no code changes.  v1 and v2 were corrupted in transit by my mail
    path, which re-wrapped the long "@@ ... @@" hunk-context lines so the
    patch would not apply.  Resent with a base64-encoded body so it is
    transmitted verbatim.  Sorry for the noise.

    v1: https://lore.kernel.org/linux-cifs/CADAuDAPSq+BUcB1SkHqkZsF364mShyE6jsaB+vk9zm=5Q+LHFw@mail.gmail.com/

 fs/smb/client/smb2misc.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/fs/smb/client/smb2misc.c b/fs/smb/client/smb2misc.c
index 2a7355ce1a07..6270b33147d2 100644
--- a/fs/smb/client/smb2misc.c
+++ b/fs/smb/client/smb2misc.c
@@ -19,6 +19,8 @@
 #include "nterr.h"
 #include "cached_dir.h"

+static unsigned int __smb2_calc_size(void *buf, bool *have_data);
+
 static int
 check_smb2_hdr(struct smb2_hdr *shdr, __u64 mid)
 {
@@ -145,6 +147,7 @@ smb2_check_message(char *buf, unsigned int
pdu_len, unsigned int len,
 	int command;
 	__u32 calc_len; /* calculated length */
 	__u64 mid;
+	bool have_data;

 	/* If server is a channel, select the primary channel */
 	pserver = SERVER_IS_CHAN(server) ? server->primary_server : server;
@@ -228,7 +231,8 @@ smb2_check_message(char *buf, unsigned int
pdu_len, unsigned int len,
 		}
 	}

-	calc_len = smb2_calc_size(buf);
+	have_data = false;
+	calc_len = __smb2_calc_size(buf, &have_data);

 	/* For SMB2_IOCTL, OutputOffset and OutputLength are optional, so might
 	 * be 0, and not a real miscalculation */
@@ -247,8 +251,13 @@ smb2_check_message(char *buf, unsigned int
pdu_len, unsigned int len,
 		/* Windows 7 server returns 24 bytes more */
 		if (calc_len + 24 == len && command == SMB2_OPLOCK_BREAK_HE)
 			return 0;
-		/* server can return one byte more due to implied bcc[0] */
-		if (calc_len == len + 1)
+		/*
+		 * Server can return one byte more due to implied bcc[0].
+		 * Allow it only when there is no data area; if data_length > 0
+		 * the +1 gap indicates an overreported data length rather than
+		 * the bcc[0] omission.
+		 */
+		if (calc_len == len + 1 && !have_data)
 			return 0;

 		/*
@@ -409,14 +418,17 @@ smb2_get_data_area_len(int *off, int *len,
struct smb2_hdr *shdr)
 /*
  * Calculate the size of the SMB message based on the fixed header
  * portion, the number of word parameters and the data portion of the message.
+ * If have_data is non-NULL, it is set to true when a non-empty data area was
+ * found (data_length > 0), allowing callers to distinguish the implied bcc[0]
+ * case (no data area) from an overreported data length.
  */
-unsigned int
-smb2_calc_size(void *buf)
+static unsigned int
+__smb2_calc_size(void *buf, bool *have_data)
 {
 	struct smb2_pdu *pdu = buf;
 	struct smb2_hdr *shdr = &pdu->hdr;
 	int offset; /* the offset from the beginning of SMB to data area */
-	int data_length; /* the length of the variable length data area */
+	int data_length = 0; /* the length of the variable length data area */
 	/* Structure Size has already been checked to make sure it is 64 */
 	int len = le16_to_cpu(shdr->StructureSize);

@@ -449,9 +461,17 @@ smb2_calc_size(void *buf)
 	}
 calc_size_exit:
 	cifs_dbg(FYI, "SMB2 len %d\n", len);
+	if (have_data)
+		*have_data = (data_length > 0);
 	return len;
 }

+unsigned int
+smb2_calc_size(void *buf)
+{
+	return __smb2_calc_size(buf, NULL);
+}
+
 /* Note: caller must free return buffer */
 __le16 *
 cifs_convert_path_to_utf16(const char *from, struct cifs_sb_info *cifs_sb)
-- 
2.52.0