Re: [PATCH] x86/kvm: fix FASTOP_SIZE when return thunks are enabled

[Paolo Bonzini]

On 7/14/22 11:52, Peter Zijlstra wrote:
> On Wed, Jul 13, 2022 at 02:12:41PM -0300, Thadeu Lima de Souza Cascardo wrote:
> > The return thunk call makes the fastop functions larger, just like IBT
> > does. Consider a 16-byte FASTOP_SIZE when CONFIG_RETHUNK is enabled.
> >
> > Otherwise, functions will be incorrectly aligned and when computing their
> > position for differently sized operators, they will executed in the middle
> > or end of a function, which may as well be an int3, leading to a crash
> > like:
>
> Bah.. I did the SETcc stuff, but then forgot about the FASTOP :/
>
>    af2e140f3420 ("x86/kvm: Fix SETcc emulation for return thunks")
>
> > Fixes: aa3d480315ba ("x86: Use return-thunk in asm code")
> > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/emulate.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index db96bf7d1122..d779eea1052e 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -190,7 +190,7 @@
> >   #define X16(x...) X8(x), X8(x)
> >   
> >   #define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
> > -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT))
> > +#define FASTOP_SIZE (8 * (1 + (HAS_KERNEL_IBT | IS_ENABLED(CONFIG_RETHUNK))))
>
> Would it make sense to do something like this instead?

Yes, definitely.  Applied with a small tweak to make FASTOP_LENGTH
more similar to SETCC_LENGTH:
 
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index db96bf7d1122..0a15b0fec6d9 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -189,8 +189,12 @@
 #define X8(x...) X4(x), X4(x)
 #define X16(x...) X8(x), X8(x)
 
-#define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
-#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT))
+#define NR_FASTOP	(ilog2(sizeof(ulong)) + 1)
+#define RET_LENGTH	(1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
+			 IS_ENABLED(CONFIG_SLS))
+#define FASTOP_LENGTH	(ENDBR_INSN_SIZE + 7 + RET_LENGTH)
+#define FASTOP_SIZE	(8 << ((FASTOP_LENGTH > 8) & 1) << ((FASTOP_LENGTH > 16) & 1))
+static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
 
 struct opcode {
 	u64 flags;
@@ -442,8 +446,6 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
  * RET | JMP __x86_return_thunk	[1,5 bytes; CONFIG_RETHUNK]
  * INT3				[1 byte; CONFIG_SLS]
  */
-#define RET_LENGTH	(1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
-			 IS_ENABLED(CONFIG_SLS))
 #define SETCC_LENGTH	(ENDBR_INSN_SIZE + 3 + RET_LENGTH)
 #define SETCC_ALIGN	(4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
 static_assert(SETCC_LENGTH <= SETCC_ALIGN);


Paolo