linux-kernel - [PATCH] qemu_fw_cfg: Make fw_cfg_rev_attr a proper kobj

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Date:   Thu, 11 Feb 2021 12:42:58 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Gabriel Somlo <somlo@....edu>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>, qemu-devel@...gnu.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] qemu_fw_cfg: Make fw_cfg_rev_attr a proper kobj_attribute

fw_cfg_showrev() is called by an indirect call in kobj_attr_show(),
which violates clang's CFI checking because fw_cfg_showrev()'s second
parameter is 'struct attribute', whereas the ->show() member of 'struct
kobj_structure' expects the second parameter to be of type 'struct
kobj_attribute'.

$ cat /sys/firmware/qemu_fw_cfg/rev
3

$ dmesg | grep "CFI failure"
[   26.016832] CFI failure (target: fw_cfg_showrev+0x0/0x8):

Fix this by converting fw_cfg_rev_attr to 'struct kobj_attribute' where
this would have been caught automatically by the incompatible pointer
types compiler warning. Update fw_cfg_showrev() accordingly.

Fixes: 75f3e8e47f38 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Link: https://github.com/ClangBuiltLinux/linux/issues/1299
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 drivers/firmware/qemu_fw_cfg.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 0078260fbabe..172c751a4f6c 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -299,15 +299,13 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static ssize_t fw_cfg_showrev(struct kobject *k, struct attribute *a, char *buf)
+static ssize_t fw_cfg_showrev(struct kobject *k, struct kobj_attribute *a,
+			      char *buf)
 {
 	return sprintf(buf, "%u\n", fw_cfg_rev);
 }
 
-static const struct {
-	struct attribute attr;
-	ssize_t (*show)(struct kobject *k, struct attribute *a, char *buf);
-} fw_cfg_rev_attr = {
+static const struct kobj_attribute fw_cfg_rev_attr = {
 	.attr = { .name = "rev", .mode = S_IRUSR },
 	.show = fw_cfg_showrev,
 };

base-commit: 92bf22614b21a2706f4993b278017e437f7785b3
-- 
2.30.1