On 25/10/24 15:16, Johannes Berg wrote:
On Fri, 2024-10-25 at 15:10 -0600, Gustavo A. R. Silva wrote:
On 25/10/24 14:48, Johannes Berg wrote:
On Fri, 2024-10-25 at 14:36 -0600, Gustavo A. R. Silva wrote:
Yeah, I was actually going to mention this commit, as it's the one that introduced
that `bool radar_detected` to the flex struct. However, it wasn't obvious to me
how `struct ieee80211_chanctx_conf conf` could overwrite `radar_detected` as I didn't
see `conf->drv_priv` being accessed through `struct struct ieee80211_chanctx_conf`.
You have to look at the drivers, see hwsim_clear_chanctx_magic() for
example; I wonder why hwsim_check_chanctx_magic() never caught this.
Sorry, I actually meant through `struct ieee80211_chanctx`. Something like:
struct ieee80211_chanctx *foo;
...
foo->conf.drv_priv[i] = something;
or
struct bar *ptr = (void *)foo->conf->drv_priv;
then write something into *ptr...
In the above cases the code will indeed overwrite `radar_detected`.
Right, that's what it does though, no? Except it doesn't have, in the
driver, "foo->conf." because mac80211 only gives it a pointer to conf,
so it's only "conf->drv_priv" (and it's the "struct bar" example.)
OK, so do you mean that pointer to `conf` is actually coming from
`foo->conf`?
Well depends what code you're looking at? I guess we should get more
concrete now. Let's say hwsim:
struct hwsim_chanctx_priv {
u32 magic;
};
...
static inline void hwsim_set_chanctx_magic(struct ieee80211_chanctx_conf *c)
{
struct hwsim_chanctx_priv *cp = (void *)c->drv_priv;
cp->magic = HWSIM_CHANCTX_MAGIC;
}
probably shouldn't be marked 'inline' now that I look at it :-)
This is being called in hwsim itself, of course:
static int mac80211_hwsim_add_chanctx(struct ieee80211_hw *hw,
struct ieee80211_chanctx_conf *ctx)
{
hwsim_set_chanctx_magic(ctx);
...
which is only referenced as a function pointer in the ops:
static const struct ieee80211_ops mac80211_hwsim_mchan_ops = {
...
.add_chanctx = mac80211_hwsim_add_chanctx,
(via some macros)
And that's called by mac80211:
static inline int drv_add_chanctx(struct ieee80211_local *local,
struct ieee80211_chanctx *ctx)
{
int ret = -EOPNOTSUPP;
might_sleep();
lockdep_assert_wiphy(local->hw.wiphy);
trace_drv_add_chanctx(local, ctx);
if (local->ops->add_chanctx)
ret = local->ops->add_chanctx(&local->hw, &ctx->conf);
so you see that the struct ieee80211_chanctx is never passed to the
driver, but instead &ctx->conf, which is the struct with the flex array
for driver priv.
So in this example, struct hwsim_chanctx_priv::magic overlaps the
radar_detected value.
(The allocation happens via chanctx_data_size.)
Ah, I see now. Thanks so much for this!
It really obscures the whole thing when pointers to flex structs
are passed to functions, and then the flex-array member is finally
accessed after a few more calls.
This is precisely why -Wfamnae is needed, because it's not that
obvious for people to immediately notice when they are introducing
this kinds of bugs.
OK, I'll send v2, shortly.
Thanks!
--
Gustavo
