[PATCH v6 3/3] fpga: microchip-spi: fix zero header_size OOB read in mpf_ops_parse_header()

mpf_ops_parse_header() reads header_size from the bitstream at
MPF_HEADER_SIZE_OFFSET (24). When header_size is zero, the expression
*(buf + header_size - 1) reads one byte before the buffer start.

Since initial_header_size is set to 71 in mpf_ops, the fpga-mgr core
guarantees the buffer is always large enough to reach MPF_HEADER_SIZE_OFFSET.
The only real gap is the zero header_size case, which cannot be resolved
by providing a larger buffer, so return -EINVAL.

Fixes: 5f8d4a9008307 ("fpga: microchip-spi: add Microchip MPF FPGA manager")
Cc: [email protected]
Signed-off-by: Sebastian Alba Vives <[email protected]>
---
Changes in v6:
  - Rebase onto linux-next. Add cover letter.
    Suggested by Xu Yilun.
Changes in v5:
  - Drop redundant count check since initial_header_size = 71 already
    guarantees the buffer covers MPF_HEADER_SIZE_OFFSET.
    Suggested by Xu Yilun.
---
 drivers/fpga/microchip-spi.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/fpga/microchip-spi.c b/drivers/fpga/microchip-spi.c
index 6134cea86..cc8f6d7bb 100644
--- a/drivers/fpga/microchip-spi.c
+++ b/drivers/fpga/microchip-spi.c
@@ -116,6 +116,9 @@ static int mpf_ops_parse_header(struct fpga_manager *mgr,
 	}
 
 	header_size = *(buf + MPF_HEADER_SIZE_OFFSET);
+	if (!header_size)
+		return -EINVAL;
+
 	if (header_size > count) {
 		info->header_size = header_size;
 		return -EAGAIN;
-- 
2.43.0