Re: [PATCH bpf-next v3 4/9] bpf/verifier: improve register value range tracking with ARSH

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Sun, Apr 22, 2018 at 09:31:19PM -0700, Yonghong Song wrote:
> 
> 
> On 4/22/18 9:19 PM, Alexei Starovoitov wrote:
> > On Sun, Apr 22, 2018 at 07:49:13PM -0700, Yonghong Song wrote:
> > > 
> > > 
> > > On 4/22/18 5:16 PM, Alexei Starovoitov wrote:
> > > > On Fri, Apr 20, 2018 at 03:18:37PM -0700, Yonghong Song wrote:
> > > > > When helpers like bpf_get_stack returns an int value
> > > > > and later on used for arithmetic computation, the LSH and ARSH
> > > > > operations are often required to get proper sign extension into
> > > > > 64-bit. For example, without this patch:
> > > > >       54: R0=inv(id=0,umax_value=800)
> > > > >       54: (bf) r8 = r0
> > > > >       55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
> > > > >       55: (67) r8 <<= 32
> > > > >       56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
> > > > >       56: (c7) r8 s>>= 32
> > > > >       57: R8=inv(id=0)
> > > > > With this patch:
> > > > >       54: R0=inv(id=0,umax_value=800)
> > > > >       54: (bf) r8 = r0
> > > > >       55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
> > > > >       55: (67) r8 <<= 32
> > > > >       56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
> > > > >       56: (c7) r8 s>>= 32
> > > > >       57: R8=inv(id=0, umax_value=800,var_off=(0x0; 0x3ff))
> > > > > With better range of "R8", later on when "R8" is added to other register,
> > > > > e.g., a map pointer or scalar-value register, the better register
> > > > > range can be derived and verifier failure may be avoided.
> > > > > 
> > > > > In our later example,
> > > > >       ......
> > > > >       usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
> > > > >       if (usize < 0)
> > > > >           return 0;
> > > > >       ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
> > > > >       ......
> > > > > Without improving ARSH value range tracking, the register representing
> > > > > "max_len - usize" will have smin_value equal to S64_MIN and will be
> > > > > rejected by verifier.
> > > > > 
> > > > > Signed-off-by: Yonghong Song <yhs@fb.com>
> > > > > ---
> > > > >    kernel/bpf/verifier.c | 26 ++++++++++++++++++++++++++
> > > > >    1 file changed, 26 insertions(+)
> > > > > 
> > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > > > index 3c8bb92..01c215d 100644
> > > > > --- a/kernel/bpf/verifier.c
> > > > > +++ b/kernel/bpf/verifier.c
> > > > > @@ -2975,6 +2975,32 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> > > > >    		/* We may learn something more from the var_off */
> > > > >    		__update_reg_bounds(dst_reg);
> > > > >    		break;
> > > > > +	case BPF_ARSH:
> > > > > +		if (umax_val >= insn_bitness) {
> > > > > +			/* Shifts greater than 31 or 63 are undefined.
> > > > > +			 * This includes shifts by a negative number.
> > > > > +			 */
> > > > > +			mark_reg_unknown(env, regs, insn->dst_reg);
> > > > > +			break;
> > > > > +		}
> > > > > +		if (dst_reg->smin_value < 0)
> > > > > +			dst_reg->smin_value >>= umin_val;
> > > > > +		else
> > > > > +			dst_reg->smin_value >>= umax_val;
> > > > > +		if (dst_reg->smax_value < 0)
> > > > > +			dst_reg->smax_value >>= umax_val;
> > > > > +		else
> > > > > +			dst_reg->smax_value >>= umin_val;
> > > > > +		if (src_known)
> > > > > +			dst_reg->var_off = tnum_rshift(dst_reg->var_off,
> > > > > +						       umin_val);
> > > > > +		else
> > > > > +			dst_reg->var_off = tnum_rshift(tnum_unknown, umin_val);
> > > > > +		dst_reg->umin_value >>= umax_val;
> > > > > +		dst_reg->umax_value >>= umin_val;
> > > > > +		/* We may learn something more from the var_off */
> > > > > +		__update_reg_bounds(dst_reg);
> > > > 
> > > > I'm struggling to understand how these bounds are computed.
> > > > Could you add examples in the comments?
> > > 
> > > Okay, let me try to add some comments for better understanding.
> > > 
> > > > In particular if dst_reg is unknown (tnum.mask == -1)
> > > > the above tnum_rshift() will clear upper bits and will make it
> > > > 64-bit positive, but that doesn't seem correct.
> > > > What am I missing?
> > > 
> > > Considering this is arith shift, we probably should just have
> > > dst_reg->var_off = tnum_unknown to be conservative.
> > > 
> > > I could miss something here as well. Let me try to write more
> > > detailed explanation, hopefully to cover all corner cases.
> > 
> > Is there a use case for !src_known ?
> 
> For typical bpf programs, the shift amount should always be known...
> If src_known is true, it must be dealing custom packets or custom
> data structures in tracing, etc.

In the example it was <<= 32 and s>>= 32 only because newly
introduced helper returns signed 32-bit integer that is later
used in the math. I have a hard time imagining useful C code that
needs arithmetic shift with a variable.