From: Jonathan Gray <jsg@jsg.id.au>
Subject: Re: Avoid implicit sign extension in vctrap()
To: Alexander Bluhm <bluhm@openbsd.org>
Cc: tech@openbsd.org
Date: Sun, 3 Aug 2025 10:57:28 +1000

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.

> 
> bluhm
> 
> > > -----------------------------------------------------------------------
> > > commit 0bdf00d115852dabeb9078497d91e19d8667898f
> > > 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..dcdfd98c909 100644
> > > --- a/sys/arch/amd64/amd64/trap.c
> > > +++ b/sys/arch/amd64/amd64/trap.c
> > > @@ -367,14 +367,14 @@ vctrap(struct trapframe *frame, int user)
> > >  			case 0xef:	/* out %ax,(%dx) */
> > >  				ghcb_sync_val(GHCB_RAX, GHCB_SZ16, &syncout);
> > >  				port = (uint16_t)frame->tf_rdx;
> > > -				sw_exitinfo1 = (port << 16) |
> > > +				sw_exitinfo1 = ((uint64_t)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;
> > > -				sw_exitinfo1 = (port << 16) |
> > > +				sw_exitinfo1 = ((uint64_t)port << 16) |
> > >  				    (1ULL << 5) | (1ULL << 0);
> > >  				frame->tf_rip += 2;
> > >  				break;
> > > @@ -386,40 +386,40 @@ vctrap(struct trapframe *frame, int user)
> > >  		case 0xe4:	/* in $port,%al */
> > >  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncin);
> > >  			port = *(rip + 1);
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 4) |
> > > +			sw_exitinfo1 = ((uint64_t)port << 16) | (1ULL << 4) |
> > >  			    (1ULL << 0);
> > >  			frame->tf_rip += 2;
> > >  			break;
> > >  		case 0xe6:	/* outb %al,$port */
> > >  			ghcb_sync_val(GHCB_RAX, GHCB_SZ8, &syncout);
> > >  			port = *(rip + 1);
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 4);
> > > +			sw_exitinfo1 = ((uint64_t)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;
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 4) |
> > > +			sw_exitinfo1 = ((uint64_t)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;
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 6) |
> > > +			sw_exitinfo1 = ((uint64_t)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;
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 4);
> > > +			sw_exitinfo1 = ((uint64_t)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;
> > > -			sw_exitinfo1 = (port << 16) | (1ULL << 6);
> > > +			sw_exitinfo1 = ((uint64_t)port << 16) | (1ULL << 6);
> > >  			frame->tf_rip += 1;
> > >  			break;
> > >  		default:
> > 
> > 
> > 
> 
>