On 24.07.24 10:36, Sascha Hauer wrote: > 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? Exactly. > I think dt being zero is a an error anyway, so what is the && size good > for? You're right. I will drop the check for v2. > >> >> 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? Will do. Thanks, Ahmad > > 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 |