[PATCH] nfc: digital: check DEP_REQ length before reading DID byte

[PATCH] nfc: digital: check DEP_REQ length before reading DID byte

[Aamir Ahmed]

digital_tg_recv_dep_req() reads resp->data[3] when the DID bit is set
in the PFB, but only sizeof(struct digital_dep_req_res) bytes (3) are
guaranteed by the preceding length check.  A crafted DEP_REQ that is
exactly 3 bytes long with the DID bit set causes an out-of-bounds read
past the valid skb data.

This read can happen on every DEP_REQ from a malicious NFC initiator
when the target device has a non-zero DID.  The stale byte is compared
against ddev->did, so in practice the mismatch causes an -EIO return,
but the read itself is undefined behavior and accesses data past the
buffer.

Reorder the condition so that the length check (resp->len < size + 1)
comes first and short-circuits before resp->data[size] is accessed.
The existing fallthrough to the later "size > resp->len" check is
preserved for the NAD case.

Fixes: 1c7a4c24fbfd ("NFC Digital: Add target NFC-DEP support")
Cc: stable@vger.kernel.org
Signed-off-by: Aamir Ahmed <elb12345@hotmail.co.uk>
---
 net/nfc/digital_dep.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/nfc/digital_dep.c b/net/nfc/digital_dep.c
index 3982fa084737..ae5407fca12d 100644
--- a/net/nfc/digital_dep.c
+++ b/net/nfc/digital_dep.c
@@ -1117,12 +1117,12 @@ static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 	pfb = dep_req->pfb;
 
 	if (DIGITAL_NFC_DEP_DID_BIT_SET(pfb)) {
-		if (ddev->did && (ddev->did == resp->data[3])) {
-			size++;
-		} else {
+		if (resp->len < size + 1 || !ddev->did ||
+		    ddev->did != resp->data[size]) {
 			rc = -EIO;
 			goto exit;
 		}
+		size++;
 	} else if (ddev->did) {
 		rc = -EIO;
 		goto exit;
-- 
2.43.0

Re: [PATCH] nfc: digital: check DEP_REQ length before reading DID byte

[Simon Horman]

On Mon, Sep 07, 2026 at 12:36:54AM +0100, Aamir Ahmed wrote:
> digital_tg_recv_dep_req() reads resp->data[3] when the DID bit is set
> in the PFB, but only sizeof(struct digital_dep_req_res) bytes (3) are
> guaranteed by the preceding length check.  A crafted DEP_REQ that is
> exactly 3 bytes long with the DID bit set causes an out-of-bounds read
> past the valid skb data.
> 
> This read can happen on every DEP_REQ from a malicious NFC initiator
> when the target device has a non-zero DID.  The stale byte is compared
> against ddev->did, so in practice the mismatch causes an -EIO return,
> but the read itself is undefined behavior and accesses data past the
> buffer.
> 
> Reorder the condition so that the length check (resp->len < size + 1)
> comes first and short-circuits before resp->data[size] is accessed.
> The existing fallthrough to the later "size > resp->len" check is
> preserved for the NAD case.
> 
> Fixes: 1c7a4c24fbfd ("NFC Digital: Add target NFC-DEP support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Aamir Ahmed <elb12345@hotmail.co.uk>

This patch looks good to me but as noted collectively by Greg and Paolo
elsewhere:
1. Please include an Assisted-by tag
2. Please keep the number of patches in-flight to 15 or less

When you repost feel free to include:

Reviewed-by: Simon Horman <horms@kernel.org>

-- 
pw-bot: changes-requested