Hi Abdelrahman,
just a remark, nothing wrong with your patch
On November 14, 2024 4:51:14 PM GMT+01:00, Abdelrahman Youssef <abdelrahmanyossef12@xxxxxxxxx> wrote:
>While fuzzing, the name marked by FDT_BEGIN_NODE sometimes extends beyond
>the struct block area, causing a heap-overflow.
>
>Since `maxlen` is an unsigned integer representing the length of name,
>It can be negative, so it overflows to large numbers, Causing strnlen()
>to overflow.
>
>So we can just change the type of maxlen to signed and check if it's a
>non-positive value, because name has a minimum length of 1 byte ('\0').
>
>Also in strnlen() we shouldn't check for bytes exceeding maxlen, so we can remove
>+ 1 in strnlen(). We also change if (len > maxlen) to >= to count for the null
>terminator.
>
>Signed-off-by: Abdelrahman Youssef <abdelrahmanyossef12@xxxxxxxxx>
>
>---
>v3 -> v4:
> - replace maxlen < 0 to maxlen <= 0 (Sascha)
> - remove + 1 in strnlen() (Sascha)
>v2 -> v3
> - changed formatting
>v1 -> v2
> - the overflow was due to integer overflow not out-of-bounds (Ahmad)
>---
> drivers/of/fdt.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
>diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>index 2c3ea31394..75af1844f3 100644
>--- a/drivers/of/fdt.c
>+++ b/drivers/of/fdt.c
>@@ -176,7 +176,7 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
> void *dt_strings;
> struct fdt_header f;
> int ret;
>- unsigned int maxlen;
>+ int maxlen;
> const struct fdt_header *fdt = infdt;
>
> ret = fdt_parse_header(infdt, size, &f);
>@@ -210,8 +210,13 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
> maxlen = (unsigned long)fdt + f.off_dt_struct +
> f.size_dt_struct - (unsigned long)name;
>
>- len = strnlen(name, maxlen + 1);
>- if (len > maxlen) {
>+ if (maxlen <= 0) {
>+ ret = -ESPIPE;
>+ goto err;
>+ }
>+
>+ len = strnlen(name, maxlen);
>+ if (len >= maxlen) {
here len cannot exceed maxlen, but if len == maxlen this means that there is not nul-byte in the name array (up to maxlen).
by not passing maxlen + 1 there is a slight behavior change but i don't know if this an issus or not.
in the previous case strnlen could read one past maxlen which is suspicious (maybe a bug)
> ret = -ESPIPE;
> goto err;
> }
