On Thu, Mar 28, 2024 at 11:01:19PM -0400, Harishankar Vishwanathan wrote: > The scalar(32)_min_max_and/or/xor functions can exhibit unsound behavior > when setting signed bounds. The following example illustrates the issue for > scalar_min_max_and(), but it applies to the other functions. > > In scalar_min_max_and() the following clause is executed when ANDing > positive numbers: > > /* ANDing two positives gives a positive, so safe to > * cast result into s64. > */ > dst_reg->smin_value = dst_reg->umin_value; > dst_reg->smax_value = dst_reg->umax_value; > > However, if umin_value and umax_value of dst_reg cross the sign boundary > (i.e., if (s64)dst_reg->umin_value > (s64)dst_reg->umax_value), then we > will end up with smin_value > smax_value, which is unsound. > > Previous works [1, 2] have discovered and reported this issue. Our tool > Agni [3] consideres it a false positive. This is because, during the > verification of the abstract operator scalar_min_max_and(), Agni restricts > its inputs to those passing through reg_bounds_sync(). This mimics > real-world verifier behavior, as reg_bounds_sync() is invariably executed > at the tail of every abstract operator. Therefore, such behavior is > unlikely in an actual verifier execution. > > However, it is still unsound for an abstract operator to exhibit behavior > where smin_value > smax_value. This patch fixes it, making the abstract > operator sound for all (well-formed) inputs. > > It's worth noting that this patch only modifies the output signed bounds > (smin/smax_value) in cases where it was previously unsound. As such, there > is no change in precision. When the unsigned bounds (umin/umax_value) cross > the sign boundary, they shouldn't be used to update the signed bounds > (smin/max_value). In only such cases, we set the output signed bounds to > unbounded [S64_MIN, S64_MAX]. We confirmed through SMT verification that > the problem occurs if and only if the unsigned bounds cross the sign > boundary. > > [1] https://sanjit-bhat.github.io/assets/pdf/ebpf-verifier-range-analysis22.pdf > [2] https://github.com/bpfverif/agni > [3] https://link.springer.com/chapter/10.1007/978-3-031-37709-9_12 > > Co-developed-by: Matan Shachnai <m.shachnai@xxxxxxxxxxx> > Signed-off-by: Matan Shachnai <m.shachnai@xxxxxxxxxxx> > Co-developed-by: Srinivas Narayana <srinivas.narayana@xxxxxxxxxxx> > Signed-off-by: Srinivas Narayana <srinivas.narayana@xxxxxxxxxxx> > Co-developed-by: Santosh Nagarakatte <santosh.nagarakatte@xxxxxxxxxxx> > Signed-off-by: Santosh Nagarakatte <santosh.nagarakatte@xxxxxxxxxxx> > Signed-off-by: Harishankar Vishwanathan <harishankar.vishwanathan@xxxxxxxxx> I'd suggest adding a fixes tag as well. This seems to go all the way back to the signed/unsigned split. Fixed: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values") Otherwise Acked-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>
