Mailing List Archive

[xen staging] x86emul: replace UB shifts
commit b6a907f8c83d37886d0523f1aeff61b98e133498
Author: Jan Beulich <jbeulich@suse.com>
AuthorDate: Fri Jul 31 17:41:58 2020 +0200
Commit: Jan Beulich <jbeulich@suse.com>
CommitDate: Fri Jul 31 17:41:58 2020 +0200

x86emul: replace UB shifts

Displacement values can be negative, hence we shouldn't left-shift them.
Or else we get

(XEN) UBSAN: Undefined behaviour in x86_emulate/x86_emulate.c:3482:55
(XEN) left shift of negative value -2

While auditing shifts, I noticed a pair of missing parentheses, which
also get added right here.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
xen/arch/x86/x86_emulate/x86_emulate.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 44474a268b..b8a4a1ca82 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -3370,7 +3370,7 @@ x86_decode(
{
generate_exception_if(d & vSIB, EXC_UD);
modrm_rm |= ((rex_prefix & 1) << 3) |
- (evex_encoded() && !evex.x) << 4;
+ ((evex_encoded() && !evex.x) << 4);
ea.type = OP_REG;
}
else if ( ad_bytes == 2 )
@@ -3417,7 +3417,7 @@ x86_decode(
ea.mem.off = insn_fetch_type(int16_t);
break;
case 1:
- ea.mem.off += insn_fetch_type(int8_t) << disp8scale;
+ ea.mem.off += insn_fetch_type(int8_t) * (1 << disp8scale);
break;
case 2:
ea.mem.off += insn_fetch_type(int16_t);
@@ -3479,7 +3479,7 @@ x86_decode(
pc_rel = mode_64bit();
break;
case 1:
- ea.mem.off += insn_fetch_type(int8_t) << disp8scale;
+ ea.mem.off += insn_fetch_type(int8_t) * (1 << disp8scale);
break;
case 2:
ea.mem.off += insn_fetch_type(int32_t);
@@ -10028,7 +10028,8 @@ x86_emulate(
continue;

rc = ops->write(ea.mem.seg,
- truncate_ea(ea.mem.off + (idx << state->sib_scale)),
+ truncate_ea(ea.mem.off +
+ idx * (1 << state->sib_scale)),
(void *)mmvalp + i * op_bytes, op_bytes, ctxt);
if ( rc != X86EMUL_OKAY )
{
@@ -10146,7 +10147,7 @@ x86_emulate(
? ops->write
: ops->read)(ea.mem.seg,
truncate_ea(ea.mem.off +
- (idx << state->sib_scale)),
+ idx * (1 << state->sib_scale)),
NULL, 0, ctxt);
if ( rc == X86EMUL_EXCEPTION )
{
--
generated by git-patchbot for /home/xen/git/xen.git#staging