[PATCH wireless v2 1/3] wifi: iwlwifi: pcie: don't infer CSME presence from a failed read

[Navon John Lukose]

iwl_pcie_check_me_status() decides whether WiAMT/CSME is present from two
register reads, without checking that either read reached the device.

iwl_read_prph() returns 0x5a5a5a5a when it cannot grab NIC access, and
that value has CNVI_SCU_REG_FOR_ECO_1_WIAMT_KNOWN set and
CNVI_SCU_REG_FOR_ECO_1_WIAMT_PRESENT clear. A read that never reached
the hardware is therefore taken as a positive statement that there is no
CSME, and the function returns without scheduling the recheck. That is
reachable at probe: iwl_pci_gen1_2_probe() carries on when
iwl_pcie_prepare_card_hw() fails, and iwl_pcie_check_me_status() then
runs against a card it cannot talk to.

The second read has the mirror-image problem: an all-ones
CSR_HW_IF_CONFIG_REG has both ME_OWN and IAMT_UP set, so a device that
has fallen off the bus latches me_present to 1. So does the one in
iwl_pcie_recheck_me_status(), which runs a second after probe with no
guarantee that the device is still answering.

me_present is never recomputed after that, and any non-zero value makes
iwl_trans_pcie_reset() downgrade IWL_RESET_MODE_PROD_RESET to
IWL_RESET_MODE_FUNC_RESET, so one bad read permanently weakens the
recovery. In the 0x5a5a5a5a case it goes the other way and permits a
product reset on a machine that may well have CSME.

Don't take those values as data. iwl_trans_is_hw_error_value() matches
0x5a5a5a5[0-f] and 0xa5a5a5a[0-f] but not ~0, so the prph read in
iwl_pcie_check_me_status() needs both tests, the way
iwl_pcie_irq_handler() does; the two CSR reads only need the ~0 one. At
probe that leaves me_present at -1 (unknown) and still schedules the
recheck; in the recheck it keeps the previous value.

This does change the reset ladder in the poisoned-read case, and -1 is
truthy: a product reset that iwl_trans_pcie_reset() used to allow -
because 0x5a5a5a5a had been read as me_present = 0 - is now downgraded
to a function level reset. That is the conservative direction, and the 0
was never a reading, but it is a behaviour change and not a no-op.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 41fff83fe6cd ("wifi: iwlwifi: pcie: check for WiAMT/CSME presence")
Signed-off-by: Navon John Lukose <navonjohnlukose@xxxxxxxxx>
---
Backport note: the bug arrived in v6.14, so the affected branches that are
still supported are 6.18.y, 7.1.y and 7.2.y. All three have the code in
pcie/gen1_2/trans.c as trans_pcie->me_present, so this applies as posted
with no rewrite.

Only if you care about anything older that has the bug - v6.14 through
v6.17, all EOL now: v6.16 and below have both functions in pcie/drv.c
(377edee91b89 "wifi: iwlwifi: pcie move gen1_2 probe to gen1_2/trans.c"
moved them), and v6.15 and below spell the field trans->me_present
(cd6d6de694e2 "wifi: iwlwifi: pcie: move ME check data to pcie" renamed
it). The hunks are otherwise identical; iwl_trans_is_hw_error_value()
exists in every affected release.

 .../net/wireless/intel/iwlwifi/pcie/gen1_2/trans.c  | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/net/wireless/intel/iwlwifi/pcie/gen1_2/trans.c b/drivers/net/wireless/intel/iwlwifi/pcie/gen1_2/trans.c
index 28b276c..c6a771e 100644
--- a/drivers/net/wireless/intel/iwlwifi/pcie/gen1_2/trans.c
+++ b/drivers/net/wireless/intel/iwlwifi/pcie/gen1_2/trans.c
@@ -4194,7 +4194,8 @@ static void iwl_pcie_recheck_me_status(struct work_struct *wk)
 	u32 val;
 
 	val = iwl_read32(trans_pcie->trans, CSR_HW_IF_CONFIG_REG);
-	trans_pcie->me_present = !!(val & CSR_HW_IF_CONFIG_REG_IAMT_UP);
+	if (val != ~0U)
+		trans_pcie->me_present = !!(val & CSR_HW_IF_CONFIG_REG_IAMT_UP);
 }
 
 static void iwl_pcie_check_me_status(struct iwl_trans *trans)
@@ -4212,15 +4213,19 @@ static void iwl_pcie_check_me_status(struct iwl_trans *trans)
 		return;
 
 	val = iwl_read_prph(trans, CNVI_SCU_REG_FOR_ECO_1);
-	if (val & CNVI_SCU_REG_FOR_ECO_1_WIAMT_KNOWN) {
+	/* iwl_read_prph() returns 0x5a5a5a5a if it never reached the NIC, and
+	 * that value has WIAMT_KNOWN set and WIAMT_PRESENT clear
+	 */
+	if (val != ~0U && !iwl_trans_is_hw_error_value(val) &&
+	    (val & CNVI_SCU_REG_FOR_ECO_1_WIAMT_KNOWN)) {
 		trans_pcie->me_present =
 			!!(val & CNVI_SCU_REG_FOR_ECO_1_WIAMT_PRESENT);
 		return;
 	}
 
 	val = iwl_read32(trans, CSR_HW_IF_CONFIG_REG);
-	if (val & (CSR_HW_IF_CONFIG_REG_ME_OWN |
-		   CSR_HW_IF_CONFIG_REG_IAMT_UP)) {
+	if (val != ~0U && (val & (CSR_HW_IF_CONFIG_REG_ME_OWN |
+				  CSR_HW_IF_CONFIG_REG_IAMT_UP))) {
 		trans_pcie->me_present = 1;
 		return;
 	}
-- 
2.55.0