[PATCH v1 1/1] arm64/insn: Avoid undefined behaviour in branch offset decode

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>]

Branch offset decoding sign-extends the immediate by shifting it left into
bit 31 and back. Perform the left shift in uint32_t and cast to int32_t
only for the final right shift to avoid UBSAN failures on negative offsets.

Fixes: 6dbf3f0e3074 ("xen/arm: arm64: Add helpers to decode and encode branch 
instructions")
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
---
 xen/arch/arm/arm64/insn.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/arm64/insn.c b/xen/arch/arm/arm64/insn.c
index 81f7914610..6b97a84ba7 100644
--- a/xen/arch/arm/arm64/insn.c
+++ b/xen/arch/arm/arm64/insn.c
@@ -225,22 +225,22 @@ u32 __kprobes aarch64_insn_gen_nop(void)
  */
 int32_t aarch64_get_branch_offset(uint32_t insn)
 {
-       int32_t imm;
+       uint32_t imm;
 
        if (aarch64_insn_is_b(insn) || aarch64_insn_is_bl(insn)) {
                imm = aarch64_insn_decode_immediate(AARCH64_INSN_IMM_26, insn);
-               return (imm << 6) >> 4;
+               return (int32_t)(imm << 6) >> 4;
        }
 
        if (aarch64_insn_is_cbz(insn) || aarch64_insn_is_cbnz(insn) ||
            aarch64_insn_is_bcond(insn)) {
                imm = aarch64_insn_decode_immediate(AARCH64_INSN_IMM_19, insn);
-               return (imm << 13) >> 11;
+               return (int32_t)(imm << 13) >> 11;
        }
 
        if (aarch64_insn_is_tbz(insn) || aarch64_insn_is_tbnz(insn)) {
                imm = aarch64_insn_decode_immediate(AARCH64_INSN_IMM_14, insn);
-               return (imm << 18) >> 16;
+               return (int32_t)(imm << 18) >> 16;
        }
 
        /* Unhandled instruction */
-- 
2.43.0