Merge branch 'bpf-fix-u32-s32-bounds-when-ranges-cross-min-max-boundary'

Eduard Zingerman says:

====================
bpf: Fix u32/s32 bounds when ranges cross min/max boundary

Cover the following cases in range refinement logic for 32-bit ranges:
- s32 range crosses U32_MAX/0 boundary, positive part of the s32 range
  overlaps with u32 range.
- s32 range crosses U32_MAX/0 boundary, negative part of the s32 range
  overlaps with u32 range.

These cases are already handled for 64-bit range refinement.

Without the fix the test in patch 2 is rejected by the verifier.
The test was reduced from sched-ext program.

Changelog:
- v2 -> v3:
  - Reverted da653de (Paul)
  - Removed !BPF_F_TEST_REG_INVARIANTS flag from
    crossing_32_bit_signed_boundary_2() (Paul)
- v1 -> v2:
  - Extended commit message and comments (Emil)
  - Targeting 'bpf' tree instead of bpf-next (Alexei)

v1: https://lore.kernel.org/bpf/9a23fbacdc6d33ec8fcb3f6988395b5129f75369.camel@gmail.com/T
v2: https://lore.kernel.org/bpf/20260305-bpf-32-bit-range-overflow-v2-0-7169206a3041@gmail.com/
---
====================

Link: https://patch.msgid.link/20260306-bpf-32-bit-range-overflow-v3-0-f7f67e060a6b@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -2511,6 +2511,30 @@ static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
 	if ((u32)reg->s32_min_value <= (u32)reg->s32_max_value) {
 		reg->u32_min_value = max_t(u32, reg->s32_min_value, reg->u32_min_value);
 		reg->u32_max_value = min_t(u32, reg->s32_max_value, reg->u32_max_value);
+	} else {
+		if (reg->u32_max_value < (u32)reg->s32_min_value) {
+			/* See __reg64_deduce_bounds() for detailed explanation.
+			 * Refine ranges in the following situation:
+			 *
+			 * 0                                                   U32_MAX
+			 * |  [xxxxxxxxxxxxxx u32 range xxxxxxxxxxxxxx]              |
+			 * |----------------------------|----------------------------|
+			 * |xxxxx s32 range xxxxxxxxx]                       [xxxxxxx|
+			 * 0                     S32_MAX S32_MIN                    -1
+			 */
+			reg->s32_min_value = (s32)reg->u32_min_value;
+			reg->u32_max_value = min_t(u32, reg->u32_max_value, reg->s32_max_value);
+		} else if ((u32)reg->s32_max_value < reg->u32_min_value) {
+			/*
+			 * 0                                                   U32_MAX
+			 * |              [xxxxxxxxxxxxxx u32 range xxxxxxxxxxxxxx]  |
+			 * |----------------------------|----------------------------|
+			 * |xxxxxxxxx]                       [xxxxxxxxxxxx s32 range |
+			 * 0                     S32_MAX S32_MIN                    -1
+			 */
+			reg->s32_max_value = (s32)reg->u32_max_value;
+			reg->u32_min_value = max_t(u32, reg->u32_min_value, reg->s32_min_value);
+		}
 	}
 }
 

tools/testing/selftests/bpf/prog_tests/reg_bounds.c

@@ -422,15 +422,69 @@ static bool is_valid_range(enum num_t t, struct range x)
 	}
 }
 
-static struct range range_improve(enum num_t t, struct range old, struct range new)
+static struct range range_intersection(enum num_t t, struct range old, struct range new)
 {
 	return range(t, max_t(t, old.a, new.a), min_t(t, old.b, new.b));
 }
 
+/*
+ * Result is precise when 'x' and 'y' overlap or form a continuous range,
+ * result is an over-approximation if 'x' and 'y' do not overlap.
+ */
+static struct range range_union(enum num_t t, struct range x, struct range y)
+{
+	if (!is_valid_range(t, x))
+		return y;
+	if (!is_valid_range(t, y))
+		return x;
+	return range(t, min_t(t, x.a, y.a), max_t(t, x.b, y.b));
+}
+
+/*
+ * This function attempts to improve x range intersecting it with y.
+ * range_cast(... to_t ...) looses precision for ranges that pass to_t
+ * min/max boundaries. To avoid such precision loses this function
+ * splits both x and y into halves corresponding to non-overflowing
+ * sub-ranges: [0, smin] and [smax, -1].
+ * Final result is computed as follows:
+ *
+ *   ((x ∩ [0, smax]) ∩ (y ∩ [0, smax])) ∪
+ *   ((x ∩ [smin,-1]) ∩ (y ∩ [smin,-1]))
+ *
+ * Precision might still be lost if final union is not a continuous range.
+ */
+static struct range range_refine_in_halves(enum num_t x_t, struct range x,
+					   enum num_t y_t, struct range y)
+{
+	struct range x_pos, x_neg, y_pos, y_neg, r_pos, r_neg;
+	u64 smax, smin, neg_one;
+
+	if (t_is_32(x_t)) {
+		smax = (u64)(u32)S32_MAX;
+		smin = (u64)(u32)S32_MIN;
+		neg_one = (u64)(u32)(s32)(-1);
+	} else {
+		smax = (u64)S64_MAX;
+		smin = (u64)S64_MIN;
+		neg_one = U64_MAX;
+	}
+	x_pos = range_intersection(x_t, x, range(x_t, 0, smax));
+	x_neg = range_intersection(x_t, x, range(x_t, smin, neg_one));
+	y_pos = range_intersection(y_t, y, range(x_t, 0, smax));
+	y_neg = range_intersection(y_t, y, range(y_t, smin, neg_one));
+	r_pos = range_intersection(x_t, x_pos, range_cast(y_t, x_t, y_pos));
+	r_neg = range_intersection(x_t, x_neg, range_cast(y_t, x_t, y_neg));
+	return range_union(x_t, r_pos, r_neg);
+
+}
+
 static struct range range_refine(enum num_t x_t, struct range x, enum num_t y_t, struct range y)
 {
 	struct range y_cast;
 
+	if (t_is_32(x_t) == t_is_32(y_t))
+		x = range_refine_in_halves(x_t, x, y_t, y);
+
 	y_cast = range_cast(y_t, x_t, y);
 
 	/* If we know that
@@ -444,7 +498,7 @@ static struct range range_refine(enum num_t x_t, struct range x, enum num_t y_t,
 	 */
 	if (x_t == S64 && y_t == S32 && y_cast.a <= S32_MAX  && y_cast.b <= S32_MAX &&
 	    (s64)x.a >= S32_MIN && (s64)x.b <= S32_MAX)
-		return range_improve(x_t, x, y_cast);
+		return range_intersection(x_t, x, y_cast);
 
 	/* the case when new range knowledge, *y*, is a 32-bit subregister
 	 * range, while previous range knowledge, *x*, is a full register
@@ -462,25 +516,11 @@ static struct range range_refine(enum num_t x_t, struct range x, enum num_t y_t,
 		x_swap = range(x_t, swap_low32(x.a, y_cast.a), swap_low32(x.b, y_cast.b));
 		if (!is_valid_range(x_t, x_swap))
 			return x;
-		return range_improve(x_t, x, x_swap);
-	}
-
-	if (!t_is_32(x_t) && !t_is_32(y_t) && x_t != y_t) {
-		if (x_t == S64 && x.a > x.b) {
-			if (x.b < y.a && x.a <= y.b)
-				return range(x_t, x.a, y.b);
-			if (x.a > y.b && x.b >= y.a)
-				return range(x_t, y.a, x.b);
-		} else if (x_t == U64 && y.a > y.b) {
-			if (y.b < x.a && y.a <= x.b)
-				return range(x_t, y.a, x.b);
-			if (y.a > x.b && y.b >= x.a)
-				return range(x_t, x.a, y.b);
-		}
+		return range_intersection(x_t, x, x_swap);
 	}
 
 	/* otherwise, plain range cast and intersection works */
-	return range_improve(x_t, x, y_cast);
+	return range_intersection(x_t, x, y_cast);
 }
 
 /* =======================

tools/testing/selftests/bpf/progs/verifier_bounds.c

@@ -1148,7 +1148,7 @@ l0_%=:	r0 = 0;						\
 SEC("xdp")
 __description("bound check with JMP32_JSLT for crossing 32-bit signed boundary")
 __success __retval(0)
-__flag(!BPF_F_TEST_REG_INVARIANTS) /* known invariants violation */
+__flag(BPF_F_TEST_REG_INVARIANTS)
 __naked void crossing_32_bit_signed_boundary_2(void)
 {
 	asm volatile ("					\
@@ -2000,4 +2000,41 @@ __naked void bounds_refinement_multiple_overlaps(void *ctx)
 	: __clobber_all);
 }
 
+SEC("socket")
+__success
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void signed_unsigned_intersection32_case1(void *ctx)
+{
+	asm volatile("									\
+	call %[bpf_get_prandom_u32];							\
+	w0 &= 0xffffffff;								\
+	if w0 < 0x3 goto 1f;		/* on fall-through u32 range [3..U32_MAX]  */	\
+	if w0 s> 0x1 goto 1f;		/* on fall-through s32 range [S32_MIN..1]  */	\
+	if w0 s< 0x0 goto 1f;		/* range can be narrowed to  [S32_MIN..-1] */	\
+	r10 = 0;			/* thus predicting the jump. */			\
+1:	exit;										\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__success
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void signed_unsigned_intersection32_case2(void *ctx)
+{
+	asm volatile("									\
+	call %[bpf_get_prandom_u32];							\
+	w0 &= 0xffffffff;								\
+	if w0 > 0x80000003 goto 1f;	/* on fall-through u32 range [0..S32_MIN+3] */	\
+	if w0 s< -3 goto 1f;		/* on fall-through s32 range [-3..S32_MAX] */	\
+	if w0 s> 5 goto 1f;		/* on fall-through s32 range [-3..5] */		\
+	if w0 <= 5 goto 1f;		/* range can be narrowed to  [0..5] */		\
+	r10 = 0;			/* thus predicting the jump */			\
+1:	exit;										\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";