From: Alexander Bluhm <bluhm@openbsd.org>
Subject: Re: [EXT] Re: Avoid implicit sign extension in vctrap()
To: tech@openbsd.org
Date: Sun, 3 Aug 2025 15:31:21 +0200

On Sun, Aug 03, 2025 at 02:41:55PM +0200, Hans-J?rg H?xer wrote:
> Hi,
> 
> On Sun, Aug 03, 2025 at 10:57:28AM +1000, Jonathan Gray wrote:
> > On Sun, Aug 03, 2025 at 02:34:20AM +0200, Alexander Bluhm wrote:
> > > On Sun, Aug 03, 2025 at 09:37:21AM +1000, Jonathan Gray wrote:
> > > > On Sat, Aug 02, 2025 at 11:24:04PM +0200, Hans-J?rg H?xer wrote:
> > > > > Hi,
> > > > > 
> > > > > found by coverty:
> > > > >     
> > > > >     port is uint16_t.  Shifting promotes to signed 32bit integer.
> > > > >     When assigning to uint64_t sw_exitinfo1 the value gets sign extended.
> > > > >     Thus when "port << 16" is greater than 0x7fffffff the upper bits
> > > > >     of sw_exitinfo1 will be all 1.  Therefore cast to uint64_t before
> > > > >     shifting.
> > > > > 
> > > > > ok?
> > > > 
> > > > or make port uint64_t and drop the casts?
> > > 
> > > We must truncate the value here.
> > > 
> > > port = (uint16_t)frame->tf_rdx;
> > > 
> > > In theory the (uint16_t) cast should do that.  But I feel safer
> > > with a 16 bit variable for that.
> > > 
> > > Or we could use an idiom without any casts.
> > > 
> > > uint64_t port = frame->tf_rdx & 0xffff;
> > 
> > I find that more readable.  I was looking for a later use of port that
> > required it to be 16-bit.  Wasn't clear it was masked with a cast.
> > 
> > The 1ULL parts could just be 1.
> 
> updated diff below.
> 
> ok?

OK bluhm@

> -----------------------------------------------------------------------------
> commit c9c614dac43c7a5d2318de7b143dcb6b4067658e
> Author: Hans-Joerg Hoexer <hshoexer@genua.de>
> Date:   Sat Aug 2 23:04:22 2025 +0200
> 
>     Avoid implicit sign extension in vctrap()
>     
>     port is uint16_t.  Shifting promotes to signed 32bit integer.  When
>     assigning to uint64_t sw_exitinfo1 the value gets sign extended.
>     Thus when "port << 16" is greater than 0x7fffffff the upper bits
>     of sw_exitinfo1 will be all 1.  Therefore apply cast to uint64_t
>     before shifting.
>     
>     Coverity CID 1648416.
> 
> diff --git a/sys/arch/amd64/amd64/trap.c b/sys/arch/amd64/amd64/trap.c
> index cb152600dca..4c32504edb9 100644
> --- a/sys/arch/amd64/amd64/trap.c
> +++ b/sys/arch/amd64/amd64/trap.c
> @@ -306,7 +306,7 @@ vctrap(struct trapframe *frame, int user)
>  {
>  	uint64_t	 sw_exitcode, sw_exitinfo1, sw_exitinfo2;
>  	uint8_t		*rip = (uint8_t *)(frame->tf_rip);
> -	uint16_t	 port;
> +	uint64_t	 port;
>  	struct ghcb_sync syncout, syncin;
>  	struct ghcb_sa	*ghcb;
>  
> @@ -366,14 +366,14 @@ vctrap(struct trapframe *frame, int user)
>  			switch (*(rip + 1)) {
>  			case 0xef:	/* out %ax,(%dx) */
>  				ghcb_sync_val(GHCB_RAX, GHCB_SZ16, &syncout);
> -				port = (uint16_t)frame->tf_rdx;
> +				port = frame->tf_rdx & 0xffff;
>  				sw_exitinfo1 = (port << 16) |
>  				    (1ULL << 5);
>  				frame->tf_rip += 2;
>  				break;
>  			case 0xed:	/* in (%dx),%ax */
>  				ghcb_sync_val(GHCB_RAX, GHCB_SZ16, &syncin);
> -				port = (uint16_t)frame->tf_rdx;
> +				port = frame->tf_rdx & 0xffff;
>  				sw_exitinfo1 = (port << 16) |
>  				    (1ULL << 5) | (1ULL << 0);
>  				frame->tf_rip += 2;
> @@ -385,40 +385,40 @@ vctrap(struct trapframe *frame, int user)
>  		    }
>  		case 0xe4:	/* in $port,%al */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncin);
> -			port = *(rip + 1);
> +			port = *(rip + 1) & 0xff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 4) |
>  			    (1ULL << 0);
>  			frame->tf_rip += 2;
>  			break;
>  		case 0xe6:	/* outb %al,$port */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncout);
> -			port = *(rip + 1);
> +			port = *(rip + 1) & 0xff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 4);
>  			frame->tf_rip += 2;
>  			break;
>  		case 0xec:	/* in (%dx),%al */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncin);
> -			port = (uint16_t)frame->tf_rdx;
> +			port = frame->tf_rdx & 0xffff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 4) |
>  			    (1ULL << 0);
>  			frame->tf_rip += 1;
>  			break;
>  		case 0xed:	/* in (%dx),%eax */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ32, &syncin);
> -			port = (uint16_t)frame->tf_rdx;
> +			port = frame->tf_rdx & 0xffff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 6) |
>  			    (1ULL << 0);
>  			frame->tf_rip += 1;
>  			break;
>  		case 0xee:	/* out %al,(%dx) */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncout);
> -			port = (uint16_t)frame->tf_rdx;
> +			port = frame->tf_rdx & 0xffff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 4);
>  			frame->tf_rip += 1;
>  			break;
>  		case 0xef:	/* out %eax,(%dx) */
>  			ghcb_sync_val(GHCB_RAX, GHCB_SZ32, &syncout);
> -			port = (uint16_t)frame->tf_rdx;
> +			port = frame->tf_rdx & 0xffff;
>  			sw_exitinfo1 = (port << 16) | (1ULL << 6);
>  			frame->tf_rip += 1;
>  			break;