[PATCH net] nfc: nci: fix use of uninitialized memory in NFC-DEP general bytes

[Muhammad Bilal <meatuni001@gmail.com>]

nci_store_general_bytes_nfc_dep() derives the length of the NFC-DEP
general bytes by subtracting the fixed general-bytes offset from the ATR
length:

  atr_res_len - NFC_ATR_RES_GT_OFFSET   (poll, offset 15)
  atr_req_len - NFC_ATR_REQ_GT_OFFSET   (listen, offset 14)

It never checks that the ATR is at least that long.  When a
RF_INTF_ACTIVATED_NTF reports an ATR shorter than the offset the
subtraction is negative; because min_t() casts its arguments to __u8 the
negative value becomes large and is then capped at
NFC_ATR_RES_GB_MAXSIZE / NFC_ATR_REQ_GB_MAXSIZE.  remote_gb_len is thus
set to up to 47/48 even though only atr_res_len/atr_req_len bytes of the
on-stack atr_res/atr_req buffer were copied from the packet, and the
following memcpy() reads the uninitialized remainder into
ndev->remote_gb.

Skip storing the general bytes (leaving remote_gb_len at 0) when the ATR
is shorter than the general-bytes offset.

Fixes: a99903ec4566 ("NFC: NCI: Handle Target mode activation")
Cc: stable@vger.kernel.org
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 net/nfc/nci/ntf.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/nfc/nci/ntf.c b/net/nfc/nci/ntf.c
index 63aa0a78472b1..802928ca4d51e 100644
--- a/net/nfc/nci/ntf.c
+++ b/net/nfc/nci/ntf.c
@@ -653,6 +653,9 @@ static int nci_store_general_bytes_nfc_dep(struct nci_dev *ndev,
 	switch (ntf->activation_rf_tech_and_mode) {
 	case NCI_NFC_A_PASSIVE_POLL_MODE:
 	case NCI_NFC_F_PASSIVE_POLL_MODE:
+		if (ntf->activation_params.poll_nfc_dep.atr_res_len <
+		    NFC_ATR_RES_GT_OFFSET)
+			break;
 		ndev->remote_gb_len = min_t(__u8,
 			(ntf->activation_params.poll_nfc_dep.atr_res_len
 						- NFC_ATR_RES_GT_OFFSET),
@@ -665,6 +668,9 @@ static int nci_store_general_bytes_nfc_dep(struct nci_dev *ndev,
 
 	case NCI_NFC_A_PASSIVE_LISTEN_MODE:
 	case NCI_NFC_F_PASSIVE_LISTEN_MODE:
+		if (ntf->activation_params.listen_nfc_dep.atr_req_len <
+		    NFC_ATR_REQ_GT_OFFSET)
+			break;
 		ndev->remote_gb_len = min_t(__u8,
 			(ntf->activation_params.listen_nfc_dep.atr_req_len
 						- NFC_ATR_REQ_GT_OFFSET),
-- 
2.54.0