[PATCH v2 2/2] of: fdt: fix oob writes with large fdt properties

[Stefan Kerkmann <s.kerkmann@pengutronix.de>]

OOB writes can be triggered when fdt->dt_size * 2 is still smaller than
the property for which memory should be allocated. This can happen under
rare circumstances when editing a fdt with the of_property command and a
property is larger than 128k in size.

This happend when editing a FIT image (which is a ftd) with the
of_property command and the Kernel image was around 8M in size.

The simplified call chain is the following:

of_property -> of_flatten_dtb -> create new fdt with 64k in size (this is
fixed) -> __of_flatten_dtb -> attempt to copy kernel image (8M) ->
fdt_ensure_space -> allocate only 128k for fdt->dt -> memcopy 8M into fdt->dt
buffer -> crash

The fix is to grow fdt->dt to hold at least the new property. The power
of 2 increment is untouched to keep the same behaviour otherwise.

Signed-off-by: Stefan Kerkmann <s.kerkmann@pengutronix.de>
---
 drivers/of/fdt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index 544294a9ac..cf08fa1cfd 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -14,6 +14,7 @@
 #include <memory.h>
 #include <linux/sizes.h>
 #include <linux/ctype.h>
+#include <linux/log2.h>
 #include <linux/overflow.h>
 #include <linux/string_helpers.h>
 #include <linux/err.h>
@@ -400,6 +401,9 @@ static int fdt_ensure_space(struct fdt *fdt, int dtsize)
 		previous = fdt->dt;
 		new_size = fdt->dt_size * 2;
 
+		if (new_size <= dtsize)
+			new_size = roundup_pow_of_two(fdt->dt_size + dtsize);
+
 		fdt->dt = memalign_realloc(previous, fdt->dt_size, new_size);
 		if (!fdt->dt) {
 			free(previous);

-- 
2.39.2