KVM: x86: fix singlestepping over syscall

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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: stable@vger.kernel.org
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

...

This commit is contained in:

Paolo Bonzini 2017-06-07 15:13:14 +02:00 • committed by Radim Krčmář

parent d6aa07c169

commit c8401dda2f

3 changed files with 34 additions and 30 deletions

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arch/x86/include/asm/kvm_emulate.h

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arch/x86/kvm/emulate.c

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62

arch/x86/kvm/x86.c

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