[PATCH 2/8] cxl/features: Bound the Get Feature output by the user output buffer

cxlctl_get_feature() allocates the output buffer from the size userspace
asked for (fwctl_rpc.out_len, arriving as *out_len), but then asks the
device for a completely independent, also userspace supplied, amount of
data:

	out_size = *out_len;
	count = le16_to_cpu(feat_in->count);
	rpc_out = kvzalloc(out_size, GFP_KERNEL);
	out_size = cxl_get_feature(..., rpc_out->payload, count, ...);

cxl_get_feature() loops until it has read @count bytes into
@rpc_out->payload, so any 'count' larger than the output allocation
overflows it, with up to 64KB of device supplied data landing past the end
of the object. An out_len of 0 additionally turns the allocation into
ZERO_SIZE_PTR.

Reject the request unless the allocation can hold the Feature data at the
offset the mailbox writes it to, i.e. sizeof(struct fwctl_rpc_cxl_out_hdr)
plus @count.

Note that struct_size_t(struct fwctl_rpc_cxl_out, payload, count) is not
the right bound here: @payload lives in a union whose largest member,
'struct cxl_mbox_get_sup_feats_out', is 8 bytes, so
sizeof(struct fwctl_rpc_cxl_out) already covers the first 8 payload bytes
and the resulting bound would reject valid requests that allocate exactly
the header plus the Feature data.

Fixes: 5908f3ed6dc2 ("cxl: Add support to handle user feature commands for get feature")
Signed-off-by: Guixin Liu <[email protected]>
---
 drivers/cxl/core/features.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/cxl/core/features.c b/drivers/cxl/core/features.c
index 0ab1a8547b7e..b631643ecc7a 100644
--- a/drivers/cxl/core/features.c
+++ b/drivers/cxl/core/features.c
@@ -471,6 +471,10 @@ static void *cxlctl_get_feature(struct cxl_features_state *cxlfs,
 	if (!count)
 		return ERR_PTR(-EINVAL);
 
+	/* cxl_get_feature() writes @count bytes at @rpc_out->payload */
+	if (out_size < sizeof(struct fwctl_rpc_cxl_out_hdr) + count)
+		return ERR_PTR(-EINVAL);
+
 	struct fwctl_rpc_cxl_out *rpc_out __free(kvfree) =
 		kvzalloc(out_size, GFP_KERNEL);
 	if (!rpc_out)
-- 
2.43.7