[Qemu-devel] [PATCH] target-arm: Avoid buffer overrun on UNPREDICTABLE ldrd/strd

[Qemu-devel] [PATCH] target-arm: Avoid buffer overrun on UNPREDICTABLE ldrd/strd

[Peter Maydell]

A LDRD or STRD where rd is not an even number is UNPREDICTABLE.
We were letting this fall through, which is OK unless rd is 15,
in which case we would attempt to do a load_reg or store_reg
to a nonexistent r16 for the second half of the double-word.
Catch the odd-numbered-rd cases and UNDEF them instead.

To do this we rearrange the structure of the code a little
so we can put the UNDEF catches at the top before we've
allocated TCG temporaries.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Spotted as a result of clang's sanitizer warning about it while
I was trying to debug a guest that had gone off the rails.

 target-arm/translate.c | 56 ++++++++++++++++++++++++++++----------------------
 1 file changed, 32 insertions(+), 24 deletions(-)

diff --git a/target-arm/translate.c b/target-arm/translate.c
index 9116529..f8f72be 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -8423,34 +8423,30 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                 }
             } else {
                 int address_offset;
-                int load;
+                bool load = insn & (1 << 20);
+                bool doubleword = false;
                 /* Misc load/store */
                 rn = (insn >> 16) & 0xf;
                 rd = (insn >> 12) & 0xf;
+
+                if (!load && (sh & 2)) {
+                    /* doubleword */
+                    ARCH(5TE);
+                    if (rd & 1) {
+                        /* UNPREDICTABLE; we choose to UNDEF */
+                        goto illegal_op;
+                    }
+                    load = (sh & 1) == 0;
+                    doubleword = true;
+                }
+
                 addr = load_reg(s, rn);
                 if (insn & (1 << 24))
                     gen_add_datah_offset(s, insn, 0, addr);
                 address_offset = 0;
-                if (insn & (1 << 20)) {
-                    /* load */
-                    tmp = tcg_temp_new_i32();
-                    switch(sh) {
-                    case 1:
-                        gen_aa32_ld16u(tmp, addr, get_mem_index(s));
-                        break;
-                    case 2:
-                        gen_aa32_ld8s(tmp, addr, get_mem_index(s));
-                        break;
-                    default:
-                    case 3:
-                        gen_aa32_ld16s(tmp, addr, get_mem_index(s));
-                        break;
-                    }
-                    load = 1;
-                } else if (sh & 2) {
-                    ARCH(5TE);
-                    /* doubleword */
-                    if (sh & 1) {
+
+                if (doubleword) {
+                    if (!load) {
                         /* store */
                         tmp = load_reg(s, rd);
                         gen_aa32_st32(tmp, addr, get_mem_index(s));
@@ -8459,7 +8455,6 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                         tmp = load_reg(s, rd + 1);
                         gen_aa32_st32(tmp, addr, get_mem_index(s));
                         tcg_temp_free_i32(tmp);
-                        load = 0;
                     } else {
                         /* load */
                         tmp = tcg_temp_new_i32();
@@ -8469,15 +8464,28 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                         tmp = tcg_temp_new_i32();
                         gen_aa32_ld32u(tmp, addr, get_mem_index(s));
                         rd++;
-                        load = 1;
                     }
                     address_offset = -4;
+                } else if (load) {
+                    /* load */
+                    tmp = tcg_temp_new_i32();
+                    switch (sh) {
+                    case 1:
+                        gen_aa32_ld16u(tmp, addr, get_mem_index(s));
+                        break;
+                    case 2:
+                        gen_aa32_ld8s(tmp, addr, get_mem_index(s));
+                        break;
+                    default:
+                    case 3:
+                        gen_aa32_ld16s(tmp, addr, get_mem_index(s));
+                        break;
+                    }
                 } else {
                     /* store */
                     tmp = load_reg(s, rd);
                     gen_aa32_st16(tmp, addr, get_mem_index(s));
                     tcg_temp_free_i32(tmp);
-                    load = 0;
                 }
                 /* Perform base writeback before the loaded value to
                    ensure correct behavior with overlapping index registers.
-- 
1.9.1