[PATCH 6.12 034/244] bpf: Fix u32/s32 bounds when ranges cross min/max boundary

Greg Kroah-Hartman
2026-03-31T16:48:42Z

From: Eduard Zingerman <eddyz87@gmail.com>

[ Upstream commit fbc7aef517d8765e4c425d2792409bb9bf2e1f13 ]

Same as in __reg64_deduce_bounds(), refine s32/u32 ranges
in __reg32_deduce_bounds() in the following situations:

- s32 range crosses U32_MAX/0 boundary, positive part of the s32 range
overlaps with u32 range:

0 U32_MAX
| [xxxxxxxxxxxxxx u32 range xxxxxxxxxxxxxx] |
|----------------------------|----------------------------|
|xxxxx s32 range xxxxxxxxx] [xxxxxxx|
0 S32_MAX S32_MIN -1

- s32 range crosses U32_MAX/0 boundary, negative part of the s32 range
overlaps with u32 range:

0 U32_MAX
| [xxxxxxxxxxxxxx u32 range xxxxxxxxxxxxxx] |
|----------------------------|----------------------------|
|xxxxxxxxx] [xxxxxxxxxxxx s32 range |
0 S32_MAX S32_MIN -1

- No refinement if ranges overlap in two intervals.

This helps for e.g. consider the following program:

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;
1: ...;

The reg_bounds.c selftest is updated to incorporate identical logic,
refinement based on non-overflowing range halves:

((x ∩ [0, smax]) ∩ (y ∩ [0, smax])) ∪
((x ∩ [smin,-1]) ∩ (y ∩ [smin,-1]))

Reported-by: Andrea Righi <arighi@nvidia.com>
Reported-by: Emil Tsalapatis <emil@etsalapatis.com>
Closes: https://lore.kernel.org/bpf/aakqucg4vcujVwif@gpd4/T/
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260306-bpf-32-bit-range-overflow-v3-1-f7f67e060a6b@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/verifier.c | 24 +++++++
.../selftests/bpf/prog_tests/reg_bounds.c | 62 +++++++++++++++++--
2 files changed, 82 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 68fa30852051e..0195726266f4d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2046,6 +2046,30 @@ 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);
+ }
}
}

diff --git a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c
index 39d42271cc460..ac4e2557f7391 100644
--- a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c
+++ b/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
@@ -462,11 +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);
+ 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);
}

/* =======================
--
2.51.0