EFI/CPER: don't dump the entire memory region

The current logic at cper_print_fw_err() doesn't check if the
error record length is big enough to handle offset. On a bad firmware,
if the ofset is above the actual record, length -= offset will
underflow, making it dump the entire memory.

The end result can be:

 - the logic taking a lot of time dumping large regions of memory;
 - data disclosure due to the memory dumps;
 - an OOPS, if it tries to dump an unmapped memory region.

Fix it by checking if the section length is too small before doing
a hex dump.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
[ rjw: Subject tweaks ]
Link: https://patch.msgid.link/1752b5ba63a3e2f148ddee813b36c996cc617e86.1767871950.git.mchehab+huawei@kernel.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: mchehab

drivers/firmware/efi/cper.c

@@ -560,6 +560,11 @@ static void cper_print_fw_err(const char *pfx,
 	} else {
 		offset = sizeof(*fw_err);
 	}
+	if (offset > length) {
+		printk("%s""error section length is too small: offset=%d, length=%d\n",
+		       pfx, offset, length);
+		return;
+	}
 
 	buf += offset;
 	length -= offset;