Re: [PATCH bpf-next] Fix latent unsoundness in and/or/xor value tracking

[Shung-Hsi Yu]

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>