bpf: Fix propagation of 32-bit signed bounds from 64-bit bounds.

The 64-bit signed bounds should not affect 32-bit signed bounds unless the
verifier knows that upper 32-bits are either all 1s or all 0s. For example the
register with smin_value==1 doesn't mean that s32_min_value is also equal to 1,
since smax_value could be larger than 32-bit subregister can hold.
The verifier refines the smax/s32_max return value from certain helpers in
do_refine_retval_range(). Teach the verifier to recognize that smin/s32_min
value is also bounded. When both smin and smax bounds fit into 32-bit
subregister the verifier can propagate those bounds.

Fixes: 3f50f132 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -1298,9 +1298,7 @@ static void __reg_combine_32_into_64(struct bpf_reg_state *reg)
 
 static bool __reg64_bound_s32(s64 a)
 {
-	if (a > S32_MIN && a < S32_MAX)
-		return true;
-	return false;
+	return a > S32_MIN && a < S32_MAX;
 }
 
 static bool __reg64_bound_u32(u64 a)
@@ -1314,10 +1312,10 @@ static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
 {
 	__mark_reg32_unbounded(reg);
 
-	if (__reg64_bound_s32(reg->smin_value))
+	if (__reg64_bound_s32(reg->smin_value) && __reg64_bound_s32(reg->smax_value)) {
 		reg->s32_min_value = (s32)reg->smin_value;
-	if (__reg64_bound_s32(reg->smax_value))
 		reg->s32_max_value = (s32)reg->smax_value;
+	}
 	if (__reg64_bound_u32(reg->umin_value))
 		reg->u32_min_value = (u32)reg->umin_value;
 	if (__reg64_bound_u32(reg->umax_value))
@@ -4895,6 +4893,8 @@ static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
 
 	ret_reg->smax_value = meta->msize_max_value;
 	ret_reg->s32_max_value = meta->msize_max_value;
+	ret_reg->smin_value = -MAX_ERRNO;
+	ret_reg->s32_min_value = -MAX_ERRNO;
 	__reg_deduce_bounds(ret_reg);
 	__reg_bound_offset(ret_reg);
 	__update_reg_bounds(ret_reg);