[PATCH net v5] qede: Fix NULL pointer dereference in TPA fragment processing

Under memory pressure, the qede driver encounters NULL pointer
dereferences when processing TPA continuation fragments.

Commit 8a8633978b84 ("qede: Add build_skb() support.") accidentally
dropped the assignment of tpa_info->buffer.data in qede_tpa_start().

When memory pressure causes an SKB allocation failure in qede_tpa_start(),
the driver sets tpa_start_fail = true and attempts to recycle the physical
page later in qede_tpa_end() via qede_reuse_page(). However, because
buffer.data was left uninitialized (NULL), qede_reuse_page() pushes a
"ghost" BD (valid DMA mapping but NULL data pointer) back into the
active Rx ring.

The next time the hardware uses this ring slot, it passes a NULL page
to qede_fill_frag_skb(), causing a kernel panic.

Example crash from production system:
 BUG: unable to handle kernel NULL pointer dereference at 0x8
 RIP: qede_fill_frag_skb+0x96/0x430 [qede]
 Call Trace:
   qede_rx_int+0xb06/0x1de0
   qede_poll+0x2f4/0x6c0
   __napi_poll+0x2d/0x130

Fix the root cause by restoring the tpa_info->buffer.data assignment
in qede_tpa_start(), ensuring valid pages are correctly tracked and
recycled. Additionally, update the stale comment for
struct qede_agg_info::buffer to reflect its current usage.

Fixes: 8a8633978b84 ("qede: Add build_skb() support.")
Suggested-by: Jakub Kicinski <[email protected]>
Cc: [email protected]
Signed-off-by: Vaibhav Nagare <[email protected]>
---
  v5: Addressed AI review feedback from Jakub Kicinski:
    - Dropped redundant fast-path hardening code (NULL checks, early exits, 
      and consume/recycle loop logic) as the root cause fix makes ghost BDs 
      mathematically unreachable.
    - Updated stale struct qede_agg_info::buffer comment in qede.h to 
      reflect its current non-preallocated usage.
    - Corrected commit title formatting in Fixes: tag.
  v4: Fix err: label handling as identified by Jakub Kicinski:
    - Restored tpa_info->buffer.data assignment in qede_tpa_start().
    - Reverted err: label in qede_tpa_end() to use tpa_start_fail flag.
  v3: Addressed AI review feedback:
    - Fixed NULL pointer recycling and array bounds check order.
  v2: Addressed AI review feedback from Simon Horman:
    - Added net_ratelimit() and fixed NULL buffer recycling.
  v1: https://lore.kernel.org/netdev/[email protected]/

 drivers/net/ethernet/qlogic/qede/qede.h    | 8 ++++----
 drivers/net/ethernet/qlogic/qede/qede_fp.c | 1 +
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/qlogic/qede/qede.h b/drivers/net/ethernet/qlogic/qede/qede.h
index 042a75f34060..0e7a0c2c1765 100644
--- a/drivers/net/ethernet/qlogic/qede/qede.h
+++ b/drivers/net/ethernet/qlogic/qede/qede.h
@@ -303,10 +303,10 @@ enum qede_agg_state {
 };
 
 struct qede_agg_info {
-	/* rx_buf is a data buffer that can be placed / consumed from rx bd
-	 * chain. It has two purposes: We will preallocate the data buffer
-	 * for each aggregation when we open the interface and will place this
-	 * buffer on the rx-bd-ring when we receive TPA_START. We don't want
+	/* buffer is used to retain the Rx consumer descriptor when a TPA
+	 * session starts. If the SKB allocation fails during TPA_START,
+	 * we use this saved buffer to safely recycle the physical page
+	 * back into the rx-bd-ring via qede_reuse_page(). We don't want
 	 * to be in a state where allocation fails, as we can't reuse the
 	 * consumer buffer in the rx-chain since FW may still be writing to it
 	 * (since header needs to be modified for TPA).
diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c b/drivers/net/ethernet/qlogic/qede/qede_fp.c
index 33e18bb69774..2d42e51ff992 100644
--- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
+++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
@@ -845,6 +845,7 @@ static void qede_tpa_start(struct qede_dev *edev,
 					      pad, false);
 	tpa_info->buffer.page_offset = sw_rx_data_cons->page_offset;
 	tpa_info->buffer.mapping = sw_rx_data_cons->mapping;
+	tpa_info->buffer.data = sw_rx_data_cons->data;
 
 	if (unlikely(!tpa_info->skb)) {
 		DP_NOTICE(edev, "Failed to allocate SKB for gro\n");
-- 
2.54.0