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Sun, 06 Sep 2026 00:04:13 -0700 (PDT) Message-ID: <55dca8a81004eba0d5ba92006e4953fc12c5d1fc.camel@gmail.com> Subject: Re: [PATCH bpf v2 2/7] bpf: Preserve packet pointer class displacement in regsafe() From: Eduard Zingerman To: Kumar Kartikeya Dwivedi , bpf@vger.kernel.org Cc: Nicholas Carlini , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Emil Tsalapatis , kkd@meta.com, kernel-team@meta.com Date: Sun, 06 Sep 2026 00:04:10 -0700 In-Reply-To: <83e2ad1e23774de204372c7e3d3bddc8f570c099.camel@gmail.com> References: <20260905083418.3723623-1-memxor@gmail.com> <20260905083418.3723623-3-memxor@gmail.com> <83e2ad1e23774de204372c7e3d3bddc8f570c099.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2-10+b1 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, 2026-09-05 at 23:40 -0700, Eduard Zingerman wrote: > On Sat, 2026-09-05 at 10:34 +0200, Kumar Kartikeya Dwivedi wrote: > > regsafe() maps packet pointer IDs between states and checks that each > > current register range is a subset of the corresponding explored > > register range. It does not, however, preserve the displacement between > > registers that share a packet pointer ID. > >=20 > > This is unsound because packet range is shared by ID. A bounds check on > > one class member updates every member, and a later access can consume t= he > > range through another member. Commit 022ac0750883 ("bpf: use reg->var_o= ff > > instead of reg->off for pointers") folded the fixed pointer offset into > > r64 and removed the old off equality check, so two individually narrowe= r > > registers can prune even when their displacement has changed. The > > explored path can then license an out-of-bounds packet access on the > > pruned path. > >=20 > > Requiring equal r64 bases would prevent the bug, but would also reject = a > > safe uniform translation of the whole class. Instead, record the base > > translation seen for the first packet pointer in each ID mapping and > > require every subsequent member to have the same translation. This keep= s > > the relative displacement invariant while retaining pruning for uniform= ly > > translated classes. Packet pointers without an ID remain unaffected. > >=20 > > Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for poi= nters") > > Reported-by: Nicholas Carlini > > Suggested-by: Nicholas Carlini > > Signed-off-by: Kumar Kartikeya Dwivedi > > --- >=20 > So, essentially this is the same problem as `id + ADD_CONST` tracking > for scalar values, where the actual difference is stored in `delta` field= . > In other words, that would be an undo of 022ac0750883 for packet pointers= . >=20 > What about the original suggested fix: >=20 > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* id = relations must be preserved */ > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (!c= heck_ids(rold->id, rcur->id, idmap)) > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return false; > =C2=A0=C2=A0=C2=A0 +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (rold->id && = rold->r64.base !=3D rcur->r64.base) > =C2=A0=C2=A0=C2=A0 +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return false; > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* new= val must satisfy old val knowledge */ > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return= range_within(rold, rcur) && > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 tnum_in(rold->var_off, rcur->var_off); >=20 > Is it that bad? > Given this logic in adjust_ptr_min_max_vals: >=20 > =C2=A0=C2=A0=C2=A0 case BPF_ADD: > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ... > if (reg_is_pkt_pointer(ptr_reg)) > if (!known) > dst_reg->id =3D ++env->id_gen; >=20 > I'd expect it to be okay. >=20 > ... In case there are veristat regressions, it seems the following should be on-par with pre 022ac0750883 state (untested, llm): --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -14685,16 +14685,20 @@ * dst_reg gets the pointer type and since some positive * integer value was added to the pointer, give it a new 'id' * if it's a PTR_TO_PACKET. - * this creates a new 'base' pointer, off_reg (variable) gets - * added into the variable offset, and we copy the fixed offset - * from ptr_reg. + * This creates a new shared base with delta zero. Constant + * arithmetic preserves the ID and accumulates delta instead; + * if delta overflows, start a new ID as well. */ dst_reg->r64 =3D cnum64_add(ptr_reg->r64, off_reg->r64); dst_reg->var_off =3D tnum_add(ptr_reg->var_off, off_reg->var_off); dst_reg->raw =3D ptr_reg->raw; if (reg_is_pkt_pointer(ptr_reg)) { - if (!known) + if (!known || + check_add_overflow(ptr_reg->delta, smin_val, + &dst_reg->delta)) { dst_reg->id =3D ++env->id_gen; + dst_reg->delta =3D 0; + } /* * Clear range for unknown addends since we can't know * where the pkt pointer ended up. Also clear AT_PKT_END / @@ -14738,8 +14742,12 @@ dst_reg->var_off =3D tnum_sub(ptr_reg->var_off, off_reg->var_off); dst_reg->raw =3D ptr_reg->raw; if (reg_is_pkt_pointer(ptr_reg)) { - if (!known) + if (!known || + check_sub_overflow(ptr_reg->delta, smin_val, + &dst_reg->delta)) { dst_reg->id =3D ++env->id_gen; + dst_reg->delta =3D 0; + } /* * Clear range if the subtrahend may be negative since * pkt pointer could move past its bounds. A positive --- a/kernel/bpf/states.c +++ b/kernel/bpf/states.c @@ -634,6 +634,9 @@ } /* id relations must be preserved */ if (!check_ids(rold->id, rcur->id, idmap)) + return false; + /* Preserve displacements from the shared packet base. */ + if (rold->id && rold->delta !=3D rcur->delta) return false; /* new val must satisfy old val knowledge */ return range_within(rold, rcur) &&