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amd64: Avoid copying td_frame from kernel procs

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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
df8dd6025a, 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:		df8dd6025a
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32057
This commit is contained in:
Mark Johnston 2021-09-25 10:15:31 -04:00
parent ed8ef7ae8b
commit ca1e447b10
1 changed files with 18 additions and 30 deletions
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48
sys/amd64/amd64/vm_machdep.c
View file

@ -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);
}