KVM: x86: fix singlestepping over syscall

TF is handled a bit differently for syscall and sysret, compared
to the other instructions: TF is checked after the instruction completes,
so that the OS can disable #DB at a syscall by adding TF to FMASK.
When the sysret is executed the #DB is taken "as if" the syscall insn
just completed.

KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
Fix the behavior, otherwise you could get #DB on a user stack which is not
nice.  This does not affect Linux guests, as they use an IST or task gate
for #DB.

This fixes CVE-2017-7518.

Cc: [email protected]
Reported-by: Andy Lutomirski <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Radim Krčmář <[email protected]>

0 files changed, 0 insertions, 0 deletions