From 76916dfa89e8900639c1055c07a295c06628a0bc Mon Sep 17 00:00:00 2001
From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 22 Jul 2024 18:29:56 +0100
Subject: [PATCH] target/arm: Avoid shifts by -1 in tszimm_shr() and
tszimm_shl()
The function tszimm_esz() returns a shift amount, or possibly -1 in
certain cases that correspond to unallocated encodings in the
instruction set. We catch these later in the trans_ functions
(generally with an "a-esz < 0" check), but before we do the
decodetree-generated code will also call tszimm_shr() or tszimm_sl(),
which will use the tszimm_esz() return value as a shift count without
checking that it is not negative, which is undefined behaviour.
Avoid the UB by checking the return value in tszimm_shr() and
tszimm_shl().
Cc: qemu-stable@nongnu.org
Resolves: Coverity CID 1547617, 1547694
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240722172957.1041231-4-peter.maydell@linaro.org
---
target/arm/tcg/translate-sve.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/target/arm/tcg/translate-sve.c b/target/arm/tcg/translate-sve.c
index 798ab2bfb1..a72c262096 100644
--- a/target/arm/tcg/translate-sve.c
+++ b/target/arm/tcg/translate-sve.c
@@ -50,13 +50,27 @@ static int tszimm_esz(DisasContext *s, int x)
static int tszimm_shr(DisasContext *s, int x)
{
- return (16 << tszimm_esz(s, x)) - x;
+ /*
+ * We won't use the tszimm_shr() value if tszimm_esz() returns -1 (the
+ * trans function will check for esz < 0), so we can return any
+ * value we like from here in that case as long as we avoid UB.
+ */
+ int esz = tszimm_esz(s, x);
+ if (esz < 0) {
+ return esz;
+ }
+ return (16 << esz) - x;
}
/* See e.g. LSL (immediate, predicated). */
static int tszimm_shl(DisasContext *s, int x)
{
- return x - (8 << tszimm_esz(s, x));
+ /* As with tszimm_shr(), value will be unused if esz < 0 */
+ int esz = tszimm_esz(s, x);
+ if (esz < 0) {
+ return esz;
+ }
+ return x - (8 << esz);
}
/* The SH bit is in bit 8. Extract the low 8 and shift. */