[PATCH net v2 1/2] nfc: llcp: avoid userspace overflow on invalid optlen

[Breno Leitao <leitao@debian.org>]

nfc_llcp_getsockopt() casts optval to (u32 __user *) for put_user(), so
the kernel always stores 4 bytes regardless of the caller-supplied
optlen. The existing min_t(u32, len, sizeof(u32)) only clamps the length
reported back to userspace; it does not constrain the store. A call with
optlen < 4 therefore writes past the user buffer, violating the
getsockopt(2) contract for all five supported optnames.

Reject any call with optlen < sizeof(u32) up front. 'len' is int, so a
plain size comparison would promote a negative optlen to size_t and slip
past the check; an explicit 'len < 0' test is added first to catch
negative values before the size compare.

Fixes: 26fd76cab2e6 ("NFC: llcp: Implement socket options")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 net/nfc/llcp_sock.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index f1be1e84f6653..aa9a78a671521 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -319,6 +319,12 @@ static int nfc_llcp_getsockopt(struct socket *sock, int level, int optname,
 	if (get_user(len, optlen))
 		return -EFAULT;
 
+	if (len < 0)
+		return -EINVAL;
+
+	if (len < sizeof(u32))
+		return -EINVAL;
+
 	local = llcp_sock->local;
 	if (!local)
 		return -ENODEV;

-- 
2.53.0-Meta