sys: add safe_read(9)

The MD function with MI interface to provide a way to read arbitrary
(canonical) KVA.  amd64 only for now.

Reviewed by:	markj
Tested by:	aokblast
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D49566

2 files changed, 26 insertions, 0 deletions

diff --git a/sys/amd64/amd64/machdep.c b/sys/amd64/amd64/machdep.c
index dbad85096a1d..db2fd5927f7f 100644
--- a/sys/amd64/amd64/machdep.c
+++ b/sys/amd64/amd64/machdep.c
@@ -1852,6 +1852,24 @@ wrmsr_early_safe_end(void)
 	memset_early(gpf_descr, 0, sizeof(*gpf_descr));
 }
 
+int
+safe_read(vm_offset_t addr, char *valp)
+{
+	struct uio uio;
+	struct iovec iov;
+
+	iov.iov_base = valp;
+	iov.iov_len = 1;
+	uio.uio_offset = addr;
+	uio.uio_iov = &iov;
+	uio.uio_iovcnt = 1;
+	uio.uio_resid = 1;
+	uio.uio_segflg = UIO_SYSSPACE;
+	uio.uio_rw = UIO_READ;
+	uio.uio_td = NULL;
+	return (uiomove_mem(UIO_MEM_KMEM, &uio));
+}
+
 #ifdef KDB
 
 /*
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
index 7f655b48ba08..88c25f06e0cb 100644
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -554,6 +554,14 @@ void	intr_prof_stack_use(struct thread *td, struct trapframe *frame);
 void counted_warning(unsigned *counter, const char *msg);
 
 /*
+ * Safely read one byte of kernel memory at address addr, placing the
+ * value into *valp.  Returns 0 on success, EFAULT if read was
+ * impossible, e.g. due to the address not being mapped or not having
+ * necessary permissions.
+ */
+int safe_read(vm_offset_t addr, char *valp);
+
+/*
  * APIs to manage deprecation and obsolescence.
  */
 void _gone_in(int major, const char *msg, ...) __printflike(2, 3);