[PATCH 4.19 607/639] firmware: dmi: Fix unlikely out-of-bounds read in save_mem_devices

[Greg Kroah-Hartman]

From: Jean Delvare <jdelvare@xxxxxxx>

[ Upstream commit 81dde26de9c08bb04c4962a15608778aaffb3cf9 ]

Before reading the Extended Size field, we should ensure it fits in
the DMI record. There is already a record length check but it does
not cover that field.

It would take a seriously corrupted DMI table to hit that bug, so no
need to worry, but we should still fix it.

Signed-off-by: Jean Delvare <jdelvare@xxxxxxx>
Fixes: 6deae96b42eb ("firmware, DMI: Add function to look up a handle and return DIMM size")
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/firmware/dmi_scan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
index f2483548cde92..0dc0c78f1fdb2 100644
--- a/drivers/firmware/dmi_scan.c
+++ b/drivers/firmware/dmi_scan.c
@@ -407,7 +407,7 @@ static void __init save_mem_devices(const struct dmi_header *dm, void *v)
 		bytes = ~0ull;
 	else if (size & 0x8000)
 		bytes = (u64)(size & 0x7fff) << 10;
-	else if (size != 0x7fff)
+	else if (size != 0x7fff || dm->length < 0x20)
 		bytes = (u64)size << 20;
 	else
 		bytes = (u64)get_unaligned((u32 *)&d[0x1C]) << 20;
-- 
2.20.1