[Devel] [PATCH vz7 v2] modules: calculate sysfs max path size at compile time to avoid static variable

[Alexander Atanasov]

in commit 3f1147ffecc3 ("ve/module: export sysfs dentries in containers")

    2) Buffer for path is made static in assumption, that modules load and unload
    my happen from time to time and there is not need to allocate this buffer each
    time we need to expose or hide module sysfs dentries (allocating on stack
    gives "the frame size of 4104 bytes is larger than 2048 bytes" warning)

This change made the path variable static to avoid large stack usage.
But its contents are searched (strchr) and modified (*sep++ = 0;) in
kernfs_perms_set. While there is lock in sysfs_set_def_perms it does not
protect the contents of the path variable just multiple calls to kernfs_perms_set.
And sysfs setup/teardown is made without holding module_mutex.
So concurrent loads/unloads can trash the variable which can result
in corrupted sysfs entries. Since snprintf is used there is no way
strchr to go beyond the end of the static buffer.

To solve this calculate max possible size of the path and allocate
it back on the stack. While when dealing with user supplied paths
using PATH_MAX is a must. But for a kernel only called function making
predictable lenght calculation is ok when no user supplied parts are used.
Do a compile time check for subdir name argument lenght and assert
if it is longer than expected.

https://jira.sw.ru/browse/PSBM-143836
Signed-off-by: Alexander Atanasov <alexander.atanasov at virtuozzo.com>
---
 kernel/module.c | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

v1->v2:
  - check moved from runtime to compile time

diff --git a/kernel/module.c b/kernel/module.c
index 8d54b575ec1f..da3aa1bc7cf4 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1795,26 +1795,39 @@ out:
 
 #ifdef CONFIG_VE
 
-static ssize_t module_sysfs_perm_set_ve(struct module *mod, char *subdir, int mask)
+static ssize_t module_sysfs_perm_set_ve_real(struct module *mod,
+						const char *subdir, int mask)
 {
-	static char path[PATH_MAX];
+	/*
+	 * To avoid using 4K stack space we can calculate max possible path
+	 * If new users came this must be updated but it will be quickly catched
+	 * since they would not see their new subdirs.
+	 * currently the only subdir is holders
+	 */
+#define VE_LONGEST_SUBDIR "holders"
+#define PERM_PATH_LEN (sizeof("module") + 2 + sizeof(VE_LONGEST_SUBDIR) + \
+				MODULE_NAME_LEN + 1)
+	char path[PERM_PATH_LEN+1];
 
 	if (snprintf(path, sizeof(path) - 1, "module/%s/%s",
 		     mod->name, (subdir) ? subdir : "") >= sizeof(path) - 1)
 		return -E2BIG;
-
 	return sysfs_set_def_perms(path, mask);
+#undef PERM_PATH_LEN
 }
 
-static ssize_t module_sysfs_hide_dir_ve(struct module *mod, char *subdir)
-{
-	return module_sysfs_perm_set_ve(mod, subdir, -1);
-}
+#define module_sysfs_perm_set_ve(mod, subdir, mask)				\
+	({									\
+		BUILD_BUG_ON_MSG(sizeof(subdir) > sizeof(VE_LONGEST_SUBDIR),	\
+			"Please, update VE_LONGEST_SUBDIR to:" # subdir);	\
+		module_sysfs_perm_set_ve_real(mod, subdir, mask);		\
+	})
 
-static ssize_t module_sysfs_expose_dir_ve(struct module *mod, char *subdir)
-{
-	return module_sysfs_perm_set_ve(mod, subdir, MAY_READ | MAY_EXEC);
-}
+
+#define module_sysfs_hide_dir_ve(mod, subdir) 					\
+		module_sysfs_perm_set_ve(mod, subdir, -1)
+#define module_sysfs_expose_dir_ve(mod, subdir) 				\
+		module_sysfs_perm_set_ve(mod, subdir, MAY_READ | MAY_EXEC)
 
 static int module_sysfs_ve_init(struct module *mod)
 {
-- 
2.31.1