[PATCH] firmware: qcom: uefisecapp: Fix overflow in GetVariable response check

[Guanglei Zhu <zhugl3@xiaopeng.com>]

qsee_uefi_get_variable() validates the response written by the UEFI
secure application before copying the variable data out of it:

	if (rsp_data->data_offset + rsp_data->data_size > rsp_data->length)
		return EFI_DEVICE_ERROR;

All three fields are u32 and come straight from the response buffer, so
the addition is performed modulo 2^32.  A response with
data_offset = 0xFFFFFFF0 and data_size = 0x20 wraps around to 0x10 and
passes the check as long as length is at least that.  The check on the
caller buffer size further down only compares data_size against
buffer_size, so a small data_size satisfies it too, and the memcpy then
reads data_size bytes from rsp_data + data_offset.

Since wrapping around requires data_offset itself to be huge, the source
pointer ends up far outside the response buffer, typically in unmapped
memory, and the fault takes the kernel down.  If the address happens to
be mapped, kernel memory is copied into the caller's buffer instead.

Reorder the check so that no addition is performed: reject data_offset
larger than length first, which also makes the subtraction below safe,
then compare data_size against the remaining room.

Fixes: 759e7a2b62eb ("firmware: Add support for Qualcomm UEFI Secure Application")
Cc: stable@vger.kernel.org
Signed-off-by: Guanglei Zhu <zhugl3@xiaopeng.com>
---
 drivers/firmware/qcom/qcom_qseecom_uefisecapp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c b/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
index befa68d1dcaf..0dda09ad156a 100644
--- a/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
+++ b/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
@@ -387,7 +387,8 @@ static efi_status_t qsee_uefi_get_variable(struct qcuefi_client *qcuefi, const e
 	if (rsp_data->length > rsp_size)
 		return EFI_DEVICE_ERROR;
 
-	if (rsp_data->data_offset + rsp_data->data_size > rsp_data->length)
+	if (rsp_data->data_offset > rsp_data->length ||
+	    rsp_data->data_size > rsp_data->length - rsp_data->data_offset)
 		return EFI_DEVICE_ERROR;
 
 	/*
-- 
2.50.1