Re: [PATCH] kobject_uevent: Fix OOB access within zap_modalias_env()

[Greg KH]

On Fri, May 24, 2024 at 01:15:01PM +0800, quic_zijuhu wrote:
> On 5/24/2024 12:33 PM, Greg KH wrote:
> > On Fri, May 24, 2024 at 12:20:03PM +0800, Zijun Hu wrote:
> >> zap_modalias_env() wrongly calculates size of memory block
> >> to move, so maybe cause OOB memory access issue, fixed by
> >> correcting size to memmove.
> > 
> > "maybe" or "does"?  That's a big difference :)
> > 
> i found this issue by reading code instead of really meeting this issue.
> this issue should be prone to happen if there are more than 1 other
> environment vars.

But does it?  Given that we have loads of memory checkers, and I haven't
ever seen any report of any overrun, it would be nice to be sure.

> do you have suggestion about term to use?

Some confirmation that this really is the case would be nice :)

> >> Fixes: 9b3fa47d4a76 ("kobject: fix suppressing modalias in uevents delivered over netlink")
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
> >> ---
> >>  lib/kobject_uevent.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> >> index 03b427e2707e..f153b4f9d4d9 100644
> >> --- a/lib/kobject_uevent.c
> >> +++ b/lib/kobject_uevent.c
> >> @@ -434,7 +434,7 @@ static void zap_modalias_env(struct kobj_uevent_env *env)
> >>  
> >>  		if (i != env->envp_idx - 1) {
> >>  			memmove(env->envp[i], env->envp[i + 1],
> >> -				env->buflen - len);
> >> +				env->buf + env->buflen - env->envp[i + 1]);
> > 
> > How is this "more correct"?  Please explain it better, this logic is not
> > obvious at all.
> > 
> env->envp[] contains pointers to env->buf[] with length env->buflen,
> we want to delete environment variable pointed by env->envp[i] with
> length @len as shown below.
> 
> env->buf[]            |-> target block <-|
> 0-----------------------------------------env->buflen
>         ^             ^
> 	| ->  @len <- |
>   env->envp[i]   env->envp[i+1]
> 
> so move "target block" forward by @len, so size of target block is
> env->buf + env->buflen - env->envp[i+1] instead of env->buflen
> -len.
> 
> do you suggest add inline comments to explain it ?

Yes please.

thanks,

greg k-h