powerpc/pseries/papr-sysparm: Expose character device to user space

Until now the papr_sysparm APIs have been kernel-internal. But user
space needs access to PAPR system parameters too. The only method
available to user space today to get or set system parameters is using
sys_rtas() and /dev/mem to pass RTAS-addressable buffers between user
space and firmware. This is incompatible with lockdown and should be
deprecated.

So provide an alternative ABI to user space in the form of a
/dev/papr-sysparm character device with just two ioctl commands (get
and set). The data payloads involved are small enough to fit in the
ioctl argument buffer, making the code relatively simple.

Exposing the system parameters through sysfs has been considered but
it would be too awkward:

* The kernel currently does not have to contain an exhaustive list of
  defined system parameters. This is a convenient property to maintain
  because we don't have to update the kernel whenever a new parameter
  is added to PAPR. Exporting a named attribute in sysfs for each
  parameter would negate this.

* Some system parameters are text-based and some are not.

* Retrieval of at least one system parameter requires input data,
  which a simple read-oriented interface can't support.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-11-e9eafd0c8c6c@linux.ibm.com

2 files changed, 69 insertions, 6 deletions

diff --git a/arch/powerpc/include/asm/papr-sysparm.h b/arch/powerpc/include/asm/papr-sysparm.h
index f5fdbd8ae9db..0dbbff59101d 100644
--- a/arch/powerpc/include/asm/papr-sysparm.h
+++ b/arch/powerpc/include/asm/papr-sysparm.h
@@ -2,8 +2,10 @@
 #ifndef _ASM_POWERPC_PAPR_SYSPARM_H
 #define _ASM_POWERPC_PAPR_SYSPARM_H
 
+#include <uapi/asm/papr-sysparm.h>
+
 typedef struct {
-	const u32 token;
+	u32 token;
 } papr_sysparm_t;
 
 #define mk_papr_sysparm(x_) ((papr_sysparm_t){ .token = x_, })
@@ -20,11 +22,14 @@ typedef struct {
 #define PAPR_SYSPARM_TLB_BLOCK_INVALIDATE_ATTRS    mk_papr_sysparm(50)
 #define PAPR_SYSPARM_LPAR_NAME                     mk_papr_sysparm(55)
 
-enum {
-	PAPR_SYSPARM_MAX_INPUT  = 1024,
-	PAPR_SYSPARM_MAX_OUTPUT = 4000,
-};
-
+/**
+ * struct papr_sysparm_buf - RTAS work area layout for system parameter functions.
+ *
+ * This is the memory layout of the buffers passed to/from
+ * ibm,get-system-parameter and ibm,set-system-parameter. It is
+ * distinct from the papr_sysparm_io_block structure that is passed
+ * between user space and the kernel.
+ */
 struct papr_sysparm_buf {
 	__be16 len;
 	char val[PAPR_SYSPARM_MAX_OUTPUT];
diff --git a/arch/powerpc/include/uapi/asm/papr-sysparm.h b/arch/powerpc/include/uapi/asm/papr-sysparm.h
new file mode 100644
index 000000000000..9f9a0f267ea5
--- /dev/null
+++ b/arch/powerpc/include/uapi/asm/papr-sysparm.h
@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_PAPR_SYSPARM_H_
+#define _UAPI_PAPR_SYSPARM_H_
+
+#include <linux/types.h>
+#include <asm/ioctl.h>
+#include <asm/papr-miscdev.h>
+
+enum {
+	PAPR_SYSPARM_MAX_INPUT  = 1024,
+	PAPR_SYSPARM_MAX_OUTPUT = 4000,
+};
+
+struct papr_sysparm_io_block {
+	__u32 parameter;
+	__u16 length;
+	char data[PAPR_SYSPARM_MAX_OUTPUT];
+};
+
+/**
+ * PAPR_SYSPARM_IOC_GET - Retrieve the value of a PAPR system parameter.
+ *
+ * Uses _IOWR because of one corner case: Retrieving the value of the
+ * "OS Service Entitlement Status" parameter (60) requires the caller
+ * to supply input data (a date string) in the buffer passed to
+ * firmware. So the @length and @data of the incoming
+ * papr_sysparm_io_block are always used to initialize the work area
+ * supplied to ibm,get-system-parameter. No other parameters are known
+ * to parameterize the result this way, and callers are encouraged
+ * (but not required) to zero-initialize @length and @data in the
+ * common case.
+ *
+ * On error the contents of the ioblock are indeterminate.
+ *
+ * Return:
+ * 0: Success; @length is the length of valid data in @data, not to exceed @PAPR_SYSPARM_MAX_OUTPUT.
+ * -EIO: Platform error. (-1)
+ * -EINVAL: Incorrect data length or format. (-9999)
+ * -EPERM: The calling partition is not allowed to access this parameter. (-9002)
+ * -EOPNOTSUPP: Parameter not supported on this platform (-3)
+ */
+#define PAPR_SYSPARM_IOC_GET _IOWR(PAPR_MISCDEV_IOC_ID, 1, struct papr_sysparm_io_block)
+
+/**
+ * PAPR_SYSPARM_IOC_SET - Update the value of a PAPR system parameter.
+ *
+ * The contents of the ioblock are unchanged regardless of success.
+ *
+ * Return:
+ * 0: Success; the parameter has been updated.
+ * -EIO: Platform error. (-1)
+ * -EINVAL: Incorrect data length or format. (-9999)
+ * -EPERM: The calling partition is not allowed to access this parameter. (-9002)
+ * -EOPNOTSUPP: Parameter not supported on this platform (-3)
+ */
+#define PAPR_SYSPARM_IOC_SET _IOW(PAPR_MISCDEV_IOC_ID, 2, struct papr_sysparm_io_block)
+
+#endif /* _UAPI_PAPR_SYSPARM_H_ */