Re: [PATCH] greybus: Fix null pointer dereference in gb_operation_response_send()

From: Suraj Sonawane
Date: Mon Oct 28 2024 - 08:46:26 EST


On 27/10/24 19:30, Alex Elder wrote:
On 10/27/24 2:53 AM, Suraj Sonawane wrote:
Fix an issue detected by the Smatch static tool:
drivers/greybus/operation.c:852 gb_operation_response_send() error:
we previously assumed 'operation->response' could be null (see line 829)

There is no need for this.  This is a case where the code is
doing something that is too involved for "smatch" to know
things are OK.

A unidirectional operation includes only a request message, but
no response message.

There are two cases:
- Unidirectional
  - There is no response buffer
  - There will be no call to gb_operation_response_alloc(),
    because the operation is unidirectional.
  - The result gets set with the errno value.  If there's
    an error (there shouldn't be), -EIO is returned.
  - We return 0 early, because it's a unidirectional operation.
- Not unidirectional
  - If there is a response, we attempt to allocate one.  If that
    fails, we return -ENOMEM early.
  - Otherwise there *is* a response (it was successfully allocated)
  - The result is set
  - It is not unidirectional, so we get a reference to the operation,
    add it to the active list (or skip to the end if not connected)
  - We record the result in the response header.  This is the line in
    question, but we know the response pointer is good.
  - We send the response.
  - On error, we drop or references and return the error code.

                    -Alex



The issue occurs because 'operation->response' may be null if the
response allocation fails at line 829. However, the code tries to
access 'operation->response->header' at line 852 without checking if
it was successfully allocated. This can cause a crash if 'response'
is null.

To fix this, add a check to ensure 'operation->response' is not null
before accessing its header. If the response is null, log an error
message and return -ENOMEM to stop further processing, preventing
any crashes or undefined behavior.

Signed-off-by: Suraj Sonawane <surajsonawane0215@xxxxxxxxx>
---
  drivers/greybus/operation.c | 8 +++++++-
  1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/greybus/operation.c b/drivers/greybus/operation.c
index 8459e9bc0..521899fbc 100644
--- a/drivers/greybus/operation.c
+++ b/drivers/greybus/operation.c
@@ -849,7 +849,13 @@ static int gb_operation_response_send(struct gb_operation *operation,
          goto err_put;
      /* Fill in the response header and send it */
-    operation->response->header->result = gb_operation_errno_map(errno);
+    if (operation->response) {
+        operation->response->header->result = gb_operation_errno_map(errno);
+    } else {
+        dev_err(&connection->hd->dev, "failed to allocate response\n");
+        ret = -ENOMEM;
+        goto err_put_active;
+    }
      ret = gb_message_send(operation->response, GFP_KERNEL);
      if (ret)


Hello Alex,

Thank you for your detailed explanation. I understand now that the existing code already handles both unidirectional and non-unidirectional cases properly, ensuring that operation->response is always allocated when needed. It seems the Smatch tool flagged this as a potential issue incorrectly.

I appreciate your insights, and I'll make sure to be more cautious about such false positives from static analysis in the future.

Thanks again for your time.

Best,
Suraj