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diff --git a/sys/amd64/amd64/support.S b/sys/amd64/amd64/support.S
--- a/sys/amd64/amd64/support.S
+++ b/sys/amd64/amd64/support.S
@@ -1581,6 +1581,26 @@
movq $wrmsr_early_faulted,(%rsp)
iretq
+ENTRY(safe_read)
+/* int safe_read(vm_offset_t address %rdi, char *valp %rsi) */
+ PUSH_FRAME_POINTER
+ movq PCPU(CURPCB),%r8
+ movq PCB_ONFAULT(%r8),%r9
+ movq $safe_read_onfault,PCB_ONFAULT(%r8)
+ .globl safe_read_read
+safe_read_read:
+ movb (%rdi),%al
+ movq %r9,PCB_ONFAULT(%r8)
+ movb %al,(%rsi)
+ xorl %eax,%eax
+ POP_FRAME_POINTER
+ ret
+safe_read_onfault:
+ movq %r9,PCB_ONFAULT(%r8)
+ movl $EFAULT,%eax
+ POP_FRAME_POINTER
+ ret
+
/*
* void pmap_pti_pcid_invalidate(uint64_t ucr3, uint64_t kcr3);
* Invalidates address space addressed by ucr3, then returns to kcr3.
diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
--- a/sys/amd64/amd64/trap.c
+++ b/sys/amd64/amd64/trap.c
@@ -351,6 +351,8 @@
},
};
+extern const char safe_read_read[];
+
/*
* Exception, fault, and trap interface to the FreeBSD kernel.
* This common code is called from assembly language IDT gate entry
@@ -791,6 +793,13 @@
p = td->td_proc;
eva = frame->tf_addr;
+ if (__predict_false(frame->tf_rip == (uintptr_t)safe_read_read &&
+ curpcb->pcb_onfault != NULL && !usermode &&
+ eva >= VM_MIN_KERNEL_ADDRESS)) {
+ frame->tf_rip = (long)curpcb->pcb_onfault;
+ return (0);
+ }
+
if (__predict_false((td->td_pflags & TDP_NOFAULTING) != 0)) {
/*
* Due to both processor errata and lazy TLB invalidation when
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -553,6 +553,14 @@
void counted_warning(unsigned *counter, const char *msg);
+/*
+ * Safely read the kernel memory at address addr, places the value
+ * into *valp. Returns 0 on success, EFAULT if read was impossible,
+ * e.g. due to the address not mapped or not having neccessary
+ * permissions.
+ */
+int safe_read(vm_offset_t addr, char *valp);
+
/*
* APIs to manage deprecation and obsolescence.
*/

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