[PATCH 07/25] bpf/validate: fix BPF_LDX | EBPF_DW signed range

[Marat Khalili <marat.khalili@huawei.com>]

Function `eval_max_load` copied signed range from unsigned regardless of
the mask (operation width) producing on 64-bit load nonsensical signed
range 0..-1 that breaks invariant min <= max relied upon in multiple
places (e.g. signed overflow detection in `eval_mul` only checks `s.min`
to make sure the range is non-negative and so on).

E.g. consider the following program with the current validation code:

    Tested program:
        0:  mov r0, #0x0
        1:  mov r3, #0x0
        2:  add r3, r1
        3:  ldxdw r2, [r3 + 16]  ; tested instruction
        4:  mov r0, #0x1
        5:  exit
    Pre-state:
       r2:  %undefined
       r3:  %buffer<24> + 0
    Post-state:
       r2:  0..-1 INTERSECT 0..UINT64_MAX (!)
       r3:  %buffer<24> + 0

Part before INTERSECT represents signed range, part after INTERSECT
represents unsigned range. Unsigned range is correctly set to full range
0..UINT64_MAX, but signed range copied from it becomes 0..-1.

Fix loading logic to only copy unsigned to signed for non-full mask.

The test will be added in subsequent commits since it depends on other
fixes.

Fixes: 8021917293d0 ("bpf: add extra validation for input BPF program")
Cc: stable@dpdk.org

Signed-off-by: Marat Khalili <marat.khalili@huawei.com>
---
 lib/bpf/bpf_validate.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/lib/bpf/bpf_validate.c b/lib/bpf/bpf_validate.c
index 41dca2fb7673..391be9cbb474 100644
--- a/lib/bpf/bpf_validate.c
+++ b/lib/bpf/bpf_validate.c
@@ -1220,10 +1220,11 @@ eval_max_load(struct bpf_reg_val *rv, uint64_t mask)
 	/* full 64-bit load */
 	if (mask == UINT64_MAX)
 		eval_smax_bound(rv, mask);
-
-	/* zero-extend load */
-	rv->s.min = rv->u.min;
-	rv->s.max = rv->u.max;
+	else {
+		/* zero-extend load */
+		rv->s.min = rv->u.min;
+		rv->s.max = rv->u.max;
+	}
 }
 
 
-- 
2.43.0