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.fatoum@xxxxxxxxxxxxxx> > --- > 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 -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |