Messages in this thread | | | Date | Fri, 12 Jan 2024 10:38:07 -0800 | Subject | Re: [PATCH v3 1/2] bpf: Reject variable offset alu on PTR_TO_FLOW_KEYS | From | Yonghong Song <> |
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On 1/12/24 7:20 AM, Hao Sun wrote: > For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off > for validation. However, variable offset ptr alu is not prohibited > for this ptr kind. So the variable offset is not checked. > > The following prog is accepted: > func#0 @0 > 0: R1=ctx() R10=fp0 > 0: (bf) r6 = r1 ; R1=ctx() R6_w=ctx() > 1: (79) r7 = *(u64 *)(r6 +144) ; R6_w=ctx() R7_w=flow_keys() > 2: (b7) r8 = 1024 ; R8_w=1024 > 3: (37) r8 /= 1 ; R8_w=scalar() > 4: (57) r8 &= 1024 ; R8_w=scalar(smin=smin32=0, > smax=umax=smax32=umax32=1024,var_off=(0x0; 0x400)) > 5: (0f) r7 += r8 > mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1 > mark_precise: frame0: regs=r8 stack= before 4: (57) r8 &= 1024 > mark_precise: frame0: regs=r8 stack= before 3: (37) r8 /= 1 > mark_precise: frame0: regs=r8 stack= before 2: (b7) r8 = 1024 > 6: R7_w=flow_keys(smin=smin32=0,smax=umax=smax32=umax32=1024,var_off > =(0x0; 0x400)) R8_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=1024, > var_off=(0x0; 0x400)) > 6: (79) r0 = *(u64 *)(r7 +0) ; R0_w=scalar() > 7: (95) exit > > This prog loads flow_keys to r7, and adds the variable offset r8 > to r7, and finally causes out-of-bounds access: > > BUG: unable to handle page fault for address: ffffc90014c80038 > ... > Call Trace: > <TASK> > bpf_dispatcher_nop_func include/linux/bpf.h:1231 [inline] > __bpf_prog_run include/linux/filter.h:651 [inline] > bpf_prog_run include/linux/filter.h:658 [inline] > bpf_prog_run_pin_on_cpu include/linux/filter.h:675 [inline] > bpf_flow_dissect+0x15f/0x350 net/core/flow_dissector.c:991 > bpf_prog_test_run_flow_dissector+0x39d/0x620 net/bpf/test_run.c:1359 > bpf_prog_test_run kernel/bpf/syscall.c:4107 [inline] > __sys_bpf+0xf8f/0x4560 kernel/bpf/syscall.c:5475 > __do_sys_bpf kernel/bpf/syscall.c:5561 [inline] > __se_sys_bpf kernel/bpf/syscall.c:5559 [inline] > __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:5559 > do_syscall_x64 arch/x86/entry/common.c:52 [inline] > do_syscall_64+0x3f/0x110 arch/x86/entry/common.c:83 > entry_SYSCALL_64_after_hwframe+0x63/0x6b > > Fix this by rejecting ptr alu with variable offset on flow_keys. > Applying the patch makes the program rejected with "R7 pointer > arithmetic on flow_keys prohibited" > > Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook") > Signed-off-by: Hao Sun <sunhao.th@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
> --- > kernel/bpf/verifier.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index adbf330d364b..65f598694d55 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -12826,6 +12826,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, > } > > switch (base_type(ptr_reg->type)) { > + case PTR_TO_FLOW_KEYS: > + if (known) > + break; > + fallthrough; > case CONST_PTR_TO_MAP: > /* smin_val represents the known value */ > if (known && smin_val == 0 && opcode == BPF_ADD)
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