tree d1bea8dad55131c4a194b55895551a1a2d38a1fe
parent edec6e015a02003c2af0ce82c54ea016b5a9e3f0
author Nick Desaulniers <ndesaulniers@google.com> 1584990763 -0700
committer Paolo Bonzini <pbonzini@redhat.com> 1584992451 -0400

KVM: VMX: don't allow memory operands for inline asm that modifies SP

THUNK_TARGET defines [thunk_target] as having "rm" input constraints
when CONFIG_RETPOLINE is not set, which isn't constrained enough for
this specific case.

For inline assembly that modifies the stack pointer before using this
input, the underspecification of constraints is dangerous, and results
in an indirect call to a previously pushed flags register.

In this case `entry`'s stack slot is good enough to satisfy the "m"
constraint in "rm", but the inline assembly in
handle_external_interrupt_irqoff() modifies the stack pointer via
push+pushf before using this input, which in this case results in
calling what was the previous state of the flags register, rather than
`entry`.

Be more specific in the constraints by requiring `entry` be in a
register, and not a memory operand.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: syzbot+3f29ca2efb056a761e38@syzkaller.appspotmail.com
Debugged-by: Alexander Potapenko <glider@google.com>
Debugged-by: Paolo Bonzini <pbonzini@redhat.com>
Debugged-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Message-Id: <20200323191243.30002-1-ndesaulniers@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>