From a150ecceeee899aac70a1dbe731675e2cb4dae18 Mon Sep 17 00:00:00 2001 From: Jan Beulich Date: Tue, 13 May 2025 14:00:41 +0200 Subject: [PATCH] x86emul: avoid UB shifts in FLDENV/FRSTOR handling 16-bit quantities, no matter whether expressed as uint16_t or as bitfield, will be promoted to plain int before doing any arithmetic on them. Shifting such values by 16 will therefore shift into the sign bit, which is UB if that bit becomes set. To account for all reads and all writes accessing opposite members of the same union, introduce yet more local variables to reduce the shift counts to 12. Fixes: be55ed744ed8 ("x86emul: support FLDENV and FRSTOR") Reported-by: Fabian Specht Signed-off-by: Jan Beulich Reviewed-by: Jason Andryuk master commit: d00fc019b6eba68bd7f2186a6427e5a005ce989f master date: 2025-04-30 08:46:21 +0200 --- xen/arch/x86/x86_emulate/blk.c | 36 +++++++++++++++++++--------------- 1 file changed, 20 insertions(+), 16 deletions(-) diff --git a/xen/arch/x86/x86_emulate/blk.c b/xen/arch/x86/x86_emulate/blk.c index e790f4f900..fe0122ddc6 100644 --- a/xen/arch/x86/x86_emulate/blk.c +++ b/xen/arch/x86/x86_emulate/blk.c @@ -81,17 +81,19 @@ int x86_emul_blk( if ( !s->rex_prefix ) { /* Convert 32-bit real/vm86 to 32-bit prot format. */ - unsigned int fip = fpstate.env.mode.real.fip_lo + - (fpstate.env.mode.real.fip_hi << 16); - unsigned int fdp = fpstate.env.mode.real.fdp_lo + - (fpstate.env.mode.real.fdp_hi << 16); + unsigned int fip = fpstate.env.mode.real.fip_lo & 0xf; + unsigned int fcs = (fpstate.env.mode.real.fip_lo >> 4) | + (fpstate.env.mode.real.fip_hi << 12); + unsigned int fdp = fpstate.env.mode.real.fdp_lo & 0xf; + unsigned int fds = (fpstate.env.mode.real.fdp_lo >> 4) | + (fpstate.env.mode.real.fdp_hi << 12); unsigned int fop = fpstate.env.mode.real.fop; - fpstate.env.mode.prot.fip = fip & 0xf; - fpstate.env.mode.prot.fcs = fip >> 4; + fpstate.env.mode.prot.fip = fip; + fpstate.env.mode.prot.fcs = fcs; fpstate.env.mode.prot.fop = fop; - fpstate.env.mode.prot.fdp = fdp & 0xf; - fpstate.env.mode.prot.fds = fdp >> 4; + fpstate.env.mode.prot.fdp = fdp; + fpstate.env.mode.prot.fds = fds; } if ( bytes == sizeof(fpstate.env) ) @@ -121,17 +123,19 @@ int x86_emul_blk( else { /* Convert 16-bit real/vm86 to 32-bit prot format. */ - unsigned int fip = env->mode.real.fip_lo + - (env->mode.real.fip_hi << 16); - unsigned int fdp = env->mode.real.fdp_lo + - (env->mode.real.fdp_hi << 16); + unsigned int fip = env->mode.real.fip_lo & 0xf; + unsigned int fcs = (env->mode.real.fip_lo >> 4) | + (env->mode.real.fip_hi << 12); + unsigned int fdp = env->mode.real.fdp_lo & 0xf; + unsigned int fds = (env->mode.real.fdp_lo >> 4) | + (env->mode.real.fdp_hi << 12); unsigned int fop = env->mode.real.fop; - fpstate.env.mode.prot.fip = fip & 0xf; - fpstate.env.mode.prot.fcs = fip >> 4; + fpstate.env.mode.prot.fip = fip; + fpstate.env.mode.prot.fcs = fcs; fpstate.env.mode.prot.fop = fop; - fpstate.env.mode.prot.fdp = fdp & 0xf; - fpstate.env.mode.prot.fds = fdp >> 4; + fpstate.env.mode.prot.fdp = fdp; + fpstate.env.mode.prot.fds = fds; } if ( bytes == sizeof(*env) )