target-arm: check that LSB <= MSB in BFI instruction

The documentation states that if LSB > MSB in BFI instruction behaviour
is unpredictable. Currently QEMU crashes because of assertion failure in
this case:

tcg/tcg-op.h:2061: tcg_gen_deposit_i32: Assertion `len <= 32' failed.

While assertion failure may meet the "unpredictable" definition this
behaviour is undesirable because it allows an unprivileged guest program
to crash the emulator with the OS and other programs.

This patch addresses the issue by throwing illegal instruction exception
if LSB > MSB. Only ARM decoder is affected because Thumb decoder already
has this check in place.

To reproduce issue run the following program

int main(void) {
    asm volatile (".long 0x07c00c12" :: );
    return 0;
}

compiled with
  gcc -marm -static badop_arm.c -o badop_arm

Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/target-arm/translate.c b/target-arm/translate.c
index bdfcdf169c..2c1c2a7085 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -8739,6 +8739,10 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                         ARCH(6T2);
                         shift = (insn >> 7) & 0x1f;
                         i = (insn >> 16) & 0x1f;
+                        if (i < shift) {
+                            /* UNPREDICTABLE; we choose to UNDEF */
+                            goto illegal_op;
+                        }
                         i = i + 1 - shift;
                         if (rm == 15) {
                             tmp = tcg_temp_new_i32();