On Mon, Jul 22, 2024 at 07:24:10PM +0200, Ahmad Fatoum wrote:
> The function dt_struct_advance() is used to advance a pointer to the next
> offset within the structure block, while checking that the result is in
> bounds.
>
> Unfortunately, the function used a signed size argument. This had the
> effect that a too-large size in the FDT wrapped around and caused the
> pointer to move backwards.
>
> This issue was found by libfuzzer which generated an FDT that
> always triggered an out-of-memory condition: One struct indicated a size
> that caused the pointer to move backwards.
>
> The resulting loop allocated memory on every iteration and eventually
> ran out.
>
> Fix this by using unsigned sizes and treating wrap around as an
> error case.
>
> Signed-off-by: Ahmad Fatoum <a.fat...@pengutronix.de>
> ---
> drivers/of/fdt.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index 8dca41990c87..237468cd8164 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -32,12 +32,13 @@ static inline bool __dt_ptr_ok(const struct fdt_header
> *fdt, const void *p,
> }
> #define dt_ptr_ok(fdt, p) __dt_ptr_ok(fdt, p, sizeof(*(p)),
> __alignof__(*(p)))
>
> -static inline uint32_t dt_struct_advance(struct fdt_header *f, uint32_t dt,
> int size)
> +static inline uint32_t dt_struct_advance(struct fdt_header *f, uint32_t dt,
> uint32_t size)
> {
> - dt += size;
> - dt = ALIGN(dt, 4);
> + if (check_add_overflow(dt, size, &dt))
> + return 0;
>
> - if (dt > f->off_dt_struct + f->size_dt_struct)
> + dt = ALIGN(dt, 4);
> + if ((!dt && size) || dt > f->off_dt_struct + f->size_dt_struct)
> return 0;
I am not sure I fully understand the newly added (!dt && size).
I think it's for the case when the initial addition results in something
like 0xfffffffe and the ALIGN(dt, 4) makes dt become 0, right?
I think dt being zero is a an error anyway, so what is the && size good
for?
>
> return dt;
When dt is zero it is returned here which will be considered an error by
the caller anyway, so it seems the (!dt && size) check doesn't add
anything.
Note we have dt_struct_advance() twice in the tree. Care to fix the
other place as well?
Sascha
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