amd64: Avoid copying td_frame from kernel procs

When creating a new thread, we unconditionally copy td_frame from the
creating thread.  For threads which never return to user mode, this is
unnecessary since td_frame just points to the base of the stack or a
random interrupt frame.

If KASAN is configured this copying may also trigger false positives
since the td_frame region may contain poisoned stack regions.  It was
not noticed before since thread0 used a dummy proc0_tf trapframe, and
kernel procs are generally created by thread0.  Since commit
df8dd6025af88a99d34f549fa9591a9b8f9b75b1, though, we call
cpu_thread_alloc(&thread0) when initializing FPU state, which
reinitializes thread0.td_frame.

Work around the problem by not copying the frame unless the copying
thread came from user mode.  While here, de-duplicate the copying and
remove redundant re(initialization) of td_frame.

Reported by:	syzbot+2ec89312bffbf38d9aec@syzkaller.appspotmail.com
Reviewed by:	kib
Fixes:		df8dd6025af8
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32057

1 files changed, 18 insertions, 30 deletions

diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
index 0bfcd03d6655..c0456c1d958c 100644
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -193,6 +193,24 @@ copy_thread(struct thread *td1, struct thread *td2)
 	td2->td_md.md_spinlock_count = 1;
 	td2->td_md.md_saved_flags = PSL_KERNEL | PSL_I;
 	pmap_thread_init_invl_gen(td2);
+
+	/*
+	 * Copy the trap frame for the return to user mode as if from a syscall.
+	 * This copies most of the user mode register values.  Some of these
+	 * registers are rewritten by cpu_set_upcall() and linux_set_upcall().
+	 */
+	if ((td1->td_proc->p_flag & P_KPROC) == 0) {
+		bcopy(td1->td_frame, td2->td_frame, sizeof(struct trapframe));
+
+		/*
+		 * If the current thread has the trap bit set (i.e. a debugger
+		 * had single stepped the process to the system call), we need
+		 * to clear the trap flag from the new frame. Otherwise, the new
+		 * thread will receive a (likely unexpected) SIGTRAP when it
+		 * executes the first instruction after returning to userland.
+		 */
+		td2->td_frame->tf_rflags &= ~PSL_T;
+	}
 }
 
 /*
@@ -236,23 +254,9 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
 	mdp2 = &p2->p_md;
 	bcopy(&p1->p_md, mdp2, sizeof(*mdp2));
 
-	/*
-	 * Copy the trap frame for the return to user mode as if from a
-	 * syscall.  This copies most of the user mode register values.
-	 */
-	td2->td_frame = (struct trapframe *)td2->td_md.md_stack_base - 1;
-	bcopy(td1->td_frame, td2->td_frame, sizeof(struct trapframe));
-
 	/* Set child return values. */
 	p2->p_sysent->sv_set_fork_retval(td2);
 
-	/*
-	 * If the parent process has the trap bit set (i.e. a debugger
-	 * had single stepped the process to the system call), we need
-	 * to clear the trap flag from the new frame.
-	 */
-	td2->td_frame->tf_rflags &= ~PSL_T;
-
 	/* As on i386, do not copy io permission bitmap. */
 	pcb2->pcb_tssp = NULL;
 
@@ -602,22 +606,6 @@ cpu_copy_thread(struct thread *td, struct thread *td0)
 {
 	copy_thread(td0, td);
 
-	/*
-	 * Copy user general-purpose registers.
-	 *
-	 * Some of these registers are rewritten by cpu_set_upcall()
-	 * and linux_set_upcall().
-	 */
-	bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
-
-	/* If the current thread has the trap bit set (i.e. a debugger had
-	 * single stepped the process to the system call), we need to clear
-	 * the trap flag from the new frame. Otherwise, the new thread will
-	 * receive a (likely unexpected) SIGTRAP when it executes the first
-	 * instruction after returning to userland.
-	 */
-	td->td_frame->tf_rflags &= ~PSL_T;
-
 	set_pcb_flags_raw(td->td_pcb, PCB_FULL_IRET);
 }