[PATCH v4 1/2] bpf: Fix undefined behavior in interpreter sdiv/smod for INT_MIN

[Jenny Guanni Qu <qguanni@gmail.com>]

The BPF interpreter's signed 32-bit division and modulo handlers use
the kernel abs() macro on s32 operands. The abs() macro documentation
(include/linux/math.h) explicitly states the result is undefined when
the input is the type minimum. When DST contains S32_MIN (0x80000000),
abs((s32)DST) triggers undefined behavior and returns S32_MIN unchanged
on arm64/x86. This value is then sign-extended to u64 as
0xFFFFFFFF80000000, causing do_div() to compute the wrong result.

The verifier's abstract interpretation (scalar32_min_max_sdiv) computes
the mathematically correct result for range tracking, creating a
verifier/interpreter mismatch that can be exploited for out-of-bounds
map value access.

Introduce __safe_abs32() which handles S32_MIN correctly by casting
to u32 before negating, avoiding signed overflow entirely. Replace
all 8 abs((s32)...) call sites in the interpreter's sdiv32/smod32
handlers.

Fixes: ec0e2da95f72 ("bpf: Support new signed div/mod instructions.")
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
---
 kernel/bpf/core.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 3ece2da55625..a620d4d6f567 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1736,6 +1736,12 @@ bool bpf_opcode_in_insntable(u8 code)
 }
 
 #ifndef CONFIG_BPF_JIT_ALWAYS_ON
+/* Safe absolute value for s32 to prevent undefined behavior for abs(S32_MIN) */
+static inline u32 __safe_abs32(s32 x)
+{
+	return x >= 0 ? (u32)x : -(u32)x;
+}
+
 /**
  *	___bpf_prog_run - run eBPF program on a given context
  *	@regs: is the array of MAX_BPF_EXT_REG eBPF pseudo-registers
@@ -1900,8 +1906,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 			DST = do_div(AX, (u32) SRC);
 			break;
 		case 1:
-			AX = abs((s32)DST);
-			AX = do_div(AX, abs((s32)SRC));
+			AX = __safe_abs32((s32)DST);
+			AX = do_div(AX, __safe_abs32((s32)SRC));
 			if ((s32)DST < 0)
 				DST = (u32)-AX;
 			else
@@ -1928,8 +1934,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 			DST = do_div(AX, (u32) IMM);
 			break;
 		case 1:
-			AX = abs((s32)DST);
-			AX = do_div(AX, abs((s32)IMM));
+			AX = __safe_abs32((s32)DST);
+			AX = do_div(AX, __safe_abs32((s32)IMM));
 			if ((s32)DST < 0)
 				DST = (u32)-AX;
 			else
@@ -1955,8 +1961,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 			DST = (u32) AX;
 			break;
 		case 1:
-			AX = abs((s32)DST);
-			do_div(AX, abs((s32)SRC));
+			AX = __safe_abs32((s32)DST);
+			do_div(AX, __safe_abs32((s32)SRC));
 			if (((s32)DST < 0) == ((s32)SRC < 0))
 				DST = (u32)AX;
 			else
@@ -1982,8 +1988,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 			DST = (u32) AX;
 			break;
 		case 1:
-			AX = abs((s32)DST);
-			do_div(AX, abs((s32)IMM));
+			AX = __safe_abs32((s32)DST);
+			do_div(AX, __safe_abs32((s32)IMM));
 			if (((s32)DST < 0) == ((s32)IMM < 0))
 				DST = (u32)AX;
 			else
-- 
2.34.1