[PATCH AUTOSEL 6.19-6.1] misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk_read()

[Sasha Levin]

From: Tuo Li <islituo@xxxxxxxxx>

[ Upstream commit ba75ecb97d3f4e95d59002c13afb6519205be6cb ]

In the function bcm_vk_read(), the pointer entry is checked, indicating
that it can be NULL. If entry is NULL and rc is set to -EMSGSIZE, the
following code may cause null-pointer dereferences:

  struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
  set_msg_id(&tmp_msg, entry->usr_msg_id);
  tmp_msg.size = entry->to_h_blks - 1;

To prevent these possible null-pointer dereferences, copy to_h_msg,
usr_msg_id, and to_h_blks from iter into temporary variables, and return
these temporary variables to the application instead of accessing them
through a potentially NULL entry.

Signed-off-by: Tuo Li <islituo@xxxxxxxxx>
Reviewed-by: Scott Branden <scott.branden@xxxxxxxxxxxx>
Link: https://patch.msgid.link/20251211063637.3987937-1-islituo@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

The buggy commit is present in stable trees 5.15 and 6.1 (among others),
confirming the bug exists in stable kernels.

## Summary

**Bug**: Guaranteed null-pointer dereference in `bcm_vk_read()` when
user provides a buffer too small for the message. `entry` is NULL but
dereferenced in the `else if (rc == -EMSGSIZE)` branch.

**Fix quality**: Small, surgical, correct. Saves needed values from
`iter` (which is valid) into temporary variables while inside the loop,
then uses those temporaries instead of the NULL `entry` pointer. The fix
is reviewed by the subsystem maintainer and merged by Greg KH.

**Risk**: Very low. The fix only changes when values are copied (from
inside the loop to the same code path), using temporary variables. No
behavioral change for the normal path.

**Impact**: Prevents a kernel crash/oops that any user of this driver
can trigger by calling `read()` with an undersized buffer.

**Scope**: Single file, ~10 meaningful lines changed. Clean and self-
contained.

**Stable criteria**: Meets all criteria:
- Obviously correct and tested (reviewed by maintainer)
- Fixes a real bug (null-pointer dereference = kernel crash)
- Fixes an important issue (kernel crash from userspace)
- Small and contained
- No new features or APIs

## Verification

- **git blame** confirmed the buggy lines (`entry->to_h_msg[0]`,
  `entry->usr_msg_id`, `entry->to_h_blks` at lines 1055, 1061, 1062)
  originate from commit `111d746bb4767a` (Jan 2021, "misc: bcm-vk: add
  VK messaging support")
- **Code trace** verified: `entry` is initialized to NULL (line 1012),
  remains NULL in the `-EMSGSIZE` path (only set to `iter` in the
  buffer-big-enough branch), and is then dereferenced in `else if (rc ==
  -EMSGSIZE)` branch — confirmed null-pointer dereference
- **git tag** confirmed the buggy commit is present in stable trees
  p-5.15 and p-6.1
- **Diff review** confirmed the fix correctly saves values from `iter`
  (valid pointer) into temporaries before leaving the loop, and uses
  those temporaries in the error path
- **Reviewed-by: Scott Branden** (subsystem maintainer) confirms
  correctness

**YES**

 drivers/misc/bcm-vk/bcm_vk_msg.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/bcm-vk/bcm_vk_msg.c b/drivers/misc/bcm-vk/bcm_vk_msg.c
index 1f42d1d5a630a..665a3888708ac 100644
--- a/drivers/misc/bcm-vk/bcm_vk_msg.c
+++ b/drivers/misc/bcm-vk/bcm_vk_msg.c
@@ -1010,6 +1010,9 @@ ssize_t bcm_vk_read(struct file *p_file,
 	struct device *dev = &vk->pdev->dev;
 	struct bcm_vk_msg_chan *chan = &vk->to_h_msg_chan;
 	struct bcm_vk_wkent *entry = NULL, *iter;
+	struct vk_msg_blk tmp_msg;
+	u32 tmp_usr_msg_id;
+	u32 tmp_blks;
 	u32 q_num;
 	u32 rsp_length;
 
@@ -1034,6 +1037,9 @@ ssize_t bcm_vk_read(struct file *p_file,
 					entry = iter;
 				} else {
 					/* buffer not big enough */
+					tmp_msg = iter->to_h_msg[0];
+					tmp_usr_msg_id = iter->usr_msg_id;
+					tmp_blks = iter->to_h_blks;
 					rc = -EMSGSIZE;
 				}
 				goto read_loop_exit;
@@ -1052,14 +1058,12 @@ ssize_t bcm_vk_read(struct file *p_file,
 
 		bcm_vk_free_wkent(dev, entry);
 	} else if (rc == -EMSGSIZE) {
-		struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
-
 		/*
 		 * in this case, return just the first block, so
 		 * that app knows what size it is looking for.
 		 */
-		set_msg_id(&tmp_msg, entry->usr_msg_id);
-		tmp_msg.size = entry->to_h_blks - 1;
+		set_msg_id(&tmp_msg, tmp_usr_msg_id);
+		tmp_msg.size = tmp_blks - 1;
 		if (copy_to_user(buf, &tmp_msg, VK_MSGQ_BLK_SIZE) != 0) {
 			dev_err(dev, "Error return 1st block in -EMSGSIZE\n");
 			rc = -EFAULT;
-- 
2.51.0