On 05/10/18 17:17, Jann Horn wrote:
> When I wrote commit 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification"), I
> assumed that, in order to emulate 64-bit arithmetic with 32-bit logic, it
> is sufficient to just truncate the output to 32 bits; and so I just moved
> the register size coercion that used to be at the start of the function to
> the end of the function.
>
> That assumption is true for almost every op, but not for 32-bit right
> shifts, because those can propagate information towards the least
> significant bit. Fix it by always truncating inputs for 32-bit ops to 32
> bits.
>
> Also get rid of the coerce_reg_to_size() after the ALU op, since that has
> no effect.
Might be worth saying something like "because src_reg is passed by value".
> Fixes: 468f6eafa6c4 ("bpf: fix 32-bit ALU op verification")
> Acked-by: Daniel Borkmann <dan...@iogearbox.net>
> Signed-off-by: Jann Horn <ja...@google.com>
> ---
Acked-by: Edward Cree <ec...@solarflare.com>
> kernel/bpf/verifier.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index bb07e74b34a2..465952a8e465 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2896,6 +2896,15 @@ static int adjust_scalar_min_max_vals(struct
> bpf_verifier_env *env,
> u64 umin_val, umax_val;
> u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
Incidentally, I don't see why this needs to be a u64 (rather than say a u8).
-Ed
>
> + if (insn_bitness == 32) {
> + /* Relevant for 32-bit RSH: Information can propagate towards
> + * LSB, so it isn't sufficient to only truncate the output to
> + * 32 bits.
> + */
> + coerce_reg_to_size(dst_reg, 4);
> + coerce_reg_to_size(&src_reg, 4);
> + }
> +
> smin_val = src_reg.smin_value;
> smax_val = src_reg.smax_value;
> umin_val = src_reg.umin_value;
> @@ -3131,7 +3140,6 @@ static int adjust_scalar_min_max_vals(struct
> bpf_verifier_env *env,
> if (BPF_CLASS(insn->code) != BPF_ALU64) {
> /* 32-bit ALU ops are (32,32)->32 */
> coerce_reg_to_size(dst_reg, 4);
> - coerce_reg_to_size(&src_reg, 4);
> }
>
> __reg_deduce_bounds(dst_reg);
