Re: [PATCH bpf-next v2 3/6] bpf: Reject a store through a fault prone pointer

On Sun, 2026-08-16 at 12:54 -0700, Eduard Zingerman wrote:
> On Fri, 2026-08-14 at 23:52 +0200, Daniel Borkmann wrote:
> > check_ptr_to_btf_access() allows the program to store before the default
> > BTF access path gets to reject a non read access. ac65c710cc64 ("bpf:
> > Reject writes through untrusted BTF pointers") closed that for a
> > PTR_UNTRUSTED pointer, but a bare PTR_TO_BTF_ID may fault on a dereference
> > just the same and is let through.
> > 
> > A BPF_LDX gets the BPF_PROBE_MEM rewrite in bpf_convert_ctx_accesses()
> > and a bad address is handled, but a BPF_STX does not and cannot, there
> > is no probed store to rewrite. The store is emitted as a plain one without
> > an exception table entry and a bad address panics the kernel.
> > 
> > A bpf_qdisc program can reach this, bpf_qdisc_btf_struct_access() permits a
> > write to Qdisc::limit and Qdisc::next_sched is a plain struct Qdisc pointer
> > which the walk turns into the compat type:
> > 
> >   struct Qdisc *next = sch->next_sched;
> > 
> >   next->limit = 1000;
> > 
> >   BUG: kernel NULL pointer dereference, address: 0000000000000014
> >   RIP: 0010:bpf_prog_c6e14e7f32c8e325_bpf_fifo_enqueue+0x3a/0x12b
> >   Code: [...] bf e8 03 00 00 <89> 7e 14 41 8b 7f 14 [...]
> >   Kernel panic - not syncing: Fatal exception in interrupt
> > 
> > Fix by widen the check to bpf_may_fault_on_deref() so that it covers both.
> > 
> > Fixes: 27ae7997a661 ("bpf: Introduce BPF_PROG_TYPE_STRUCT_OPS")
> > Signed-off-by: Daniel Borkmann <[email protected]>
> > ---
> >  v1 -> v2:
> >    - new patch
> > 
> >  kernel/bpf/verifier.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2e6992569187..6610e2437047 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5789,7 +5789,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
> >  		return -EACCES;
> >  	}
> >  
> > -	if (atype != BPF_READ && (type_flag(reg->type) & PTR_UNTRUSTED)) {
> > +	if (atype != BPF_READ && bpf_may_fault_on_deref(reg->type)) {
> 
> The change is correct, but it appears that the if condition could be
> simplified as just 'atype != BPF_READ'.
> The function is named check_ptr_to_btf_access() and it is only called
> when reg->type == PTR_TO_BTF_ID.

Nah, scratch that, callers check base type.
Sorry for the noise.

Acked-by: Eduard Zingerman <[email protected]>