[PATCH] ksmbd: accept share config responses from ksmbd-tools before 3.5.3

[Michail Andreas Michalopoulos <mike.michalo@gmail.com>]

ksmbd-tools releases before 3.5.2 do not have the payload_sz field in
struct ksmbd_share_config_response; the field occupies the last word of
their reserved[] area, so the kernel always sees payload_sz == 0 from
them.  ipc_validate_msg() and share_config_request() used to treat
payload_sz == 0 as "legacy userspace" and skip the payload size checks.

That case was removed by
commit f25e93768fcc ("ksmbd: prevent out-of-bounds reads in share config responses").
With payload_sz == 0:

 - the IPC$ (pipe) response fails the final message size comparison:
   these tools always send a one-byte payload, so msg_sz is
   sizeof(*resp) + 1, but the expected size is now computed as
   sizeof(*resp) + 0;
 - every other share fails "payload_sz <= veto_list_sz".

In addition, ksmbd-tools before 3.5.3 copy a global "veto files" setting
into the IPC$ response, so with such a configuration IPC$ also fails the
now unconditional "veto_list_sz > payload_sz" check, even though pipe
shares never use a veto list.

ksmbd_ipc_share_config_request() then returns NULL and every tree
connect fails with STATUS_BAD_NETWORK_NAME, so ksmbd cannot serve any
share when used with these ksmbd-tools releases, for example the 3.5.1
package shipped by Ubuntu 24.04.  Login still works, which makes the
failure look like a configuration problem:

  CIFS: VFS: BAD_NETWORK_NAME: \\127.0.0.1\IPC$
  CIFS: VFS: \\127.0.0.1 failed to connect to IPC (rc=-2)

When payload_sz is zero, derive it from the size of the message actually
received and store it back in the response, so that the checks in
ipc_validate_msg() and the path and veto list parsing in
share_config_request() all use the same bound.  The response is the
kernel's own copy made by handle_response(), and msg_sz has already been
checked to be at least sizeof(*resp), so this cannot underflow.  All
checks added by that commit then apply unchanged, bounded by the bytes
that really arrived, and newer ksmbd-tools that fill in payload_sz are
unaffected.  Also skip the veto list size check for pipe shares:
share_config_request() reads neither the path nor the veto list for
them.

Tested in QEMU with ksmbd-tools 3.5.1 and 3.5.7 and the in-kernel SMB
client over loopback, with and without a global "veto files" setting.

Fixes: f25e93768fcc ("ksmbd: prevent out-of-bounds reads in share config responses")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Michail Andreas Michalopoulos <mike.michalo@gmail.com>
---
Found with AI assistance.

f25e93768fcc was also backported to 5.15.222, 6.1.189, 6.6.158,
6.12.111, 6.18.53 and 7.2.6, so those stable lines are affected too;
this patch applies to all of them (5.15 with the older fs/ksmbd/
paths) and to ksmbd-for-next.

It keeps that commit's OOB fix: the payload size is taken from the bytes
actually received, so a short response like the one in Farhad's report
is still rejected.

Tested in QEMU (KASAN) on 7.3 and on ksmbd-for-next (1db57ad99d46) with
the in-kernel SMB client over loopback, using ksmbd-tools 3.5.1 and
3.5.7, with and without "veto files" in [global]. Unpatched, 3.5.1
fails to connect to every share; patched, all combinations mount, IPC$
connects, reads/writes work and veto files are still enforced.

 fs/smb/server/transport_ipc.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/smb/server/transport_ipc.c b/fs/smb/server/transport_ipc.c
index e550aa41ad..f5cfc32050 100644
--- a/fs/smb/server/transport_ipc.c
+++ b/fs/smb/server/transport_ipc.c
@@ -532,11 +532,25 @@ static int ipc_validate_msg(struct ipc_msg_table_entry *entry)
 		if (entry->msg_sz < sizeof(struct ksmbd_share_config_response))
 			return -EINVAL;
 
+		/*
+		 * ksmbd-tools releases before 3.5.2 leave payload_sz zero.
+		 * Derive it from the size of the message actually received so
+		 * the checks below bound the payload for those releases too.
+		 */
+		if (!resp->payload_sz)
+			resp->payload_sz = entry->msg_sz -
+				sizeof(struct ksmbd_share_config_response);
+
 		if (strnlen(resp->share_name, sizeof(resp->share_name)) ==
 		    sizeof(resp->share_name))
 			return -EINVAL;
 
-		if (resp->veto_list_sz > resp->payload_sz)
+		/*
+		 * Pipe shares carry no veto list, but ksmbd-tools before 3.5.3
+		 * copy a global "veto files" setting into the IPC$ response.
+		 */
+		if (!(resp->flags & KSMBD_SHARE_FLAG_PIPE) &&
+		    resp->veto_list_sz > resp->payload_sz)
 			return -EINVAL;
 
 		if (resp->flags != KSMBD_SHARE_FLAG_INVALID &&
-- 
2.43.0