[PATCH 1/3] HID: core: ensure the allocated report buffer can contain the reserved report ID

[Benjamin Tissoires <bentiss@kernel.org>]

When the report ID is not used, the low level transport drivers expect
the first byte to be 0. However, currently the allocated buffer not
account for that extra byte, meaning that instead of having 8 guaranteed
bytes for implement to be working, we only have 7.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Closes: https://lore.kernel.org/linux-input/c75433e0-9b47-4072-bbe8-b1d14ea97b13@rowland.harvard.edu/
Cc: stable@vger.kernel.org
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
---
 drivers/hid/hid-core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index b348d0464314ca331da073128f0ec4e0a6a91ed1..1a231dd9e4bc83202f2cbcd8b3a21e8c82b9deec 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -1883,9 +1883,12 @@ u8 *hid_alloc_report_buf(struct hid_report *report, gfp_t flags)
 	/*
 	 * 7 extra bytes are necessary to achieve proper functionality
 	 * of implement() working on 8 byte chunks
+	 * 1 extra byte for the report ID if it is null (not used) so
+	 * we can reserve that extra byte in the first position of the buffer
+	 * when sending it to .raw_request()
 	 */
 
-	u32 len = hid_report_len(report) + 7;
+	u32 len = hid_report_len(report) + 7 + (report->id == 0);
 
 	return kzalloc(len, flags);
 }

-- 
2.49.0