[PATCH] HID: bpf: Fix signedness bug in hid_bpf_hw_request

[Guangshuo Li <lgs201920130244@gmail.com>]

hid_bpf_hw_request() clamps the return value of hid_hw_raw_request() to
the size of the caller supplied buffer before copying data back to the
BPF buffer.

However, ret is signed while size is unsigned. If hid_hw_raw_request()
returns a negative error code, the comparison promotes ret to size_t.
This makes the negative value look like a very large positive value, so
the error is clamped to size. The following memcpy() then treats the
failed request as a successful transfer and copies stale data back to
the caller.

Only clamp positive return values. This preserves negative error codes
while still preventing oversized successful returns from overflowing the
caller supplied buffer.

Fixes: 2b658c1c442e ("HID: bpf: prevent buffer overflow in hid_hw_request")
Signed-off-by: Guangshuo Li <lgs201920130244@gmail.com>
---
 drivers/hid/bpf/hid_bpf_dispatch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/bpf/hid_bpf_dispatch.c b/drivers/hid/bpf/hid_bpf_dispatch.c
index d0130658091b..b13f911e5944 100644
--- a/drivers/hid/bpf/hid_bpf_dispatch.c
+++ b/drivers/hid/bpf/hid_bpf_dispatch.c
@@ -446,7 +446,7 @@ hid_bpf_hw_request(struct hid_bpf_ctx *ctx, __u8 *buf, size_t buf__sz,
 					      (u64)(long)ctx,
 					      true); /* prevent infinite recursions */
 
-	if (ret > size)
+	if (ret > 0 && ret > size)
 		ret = size;
 	if (ret > 0)
 		memcpy(buf, dma_data, ret);
-- 
2.43.0