vfs: allocate page instead of names_cache buffer in mount_block_root

First, it's incorrect to call putname() after __getname_gfp() since the
bare __getname_gfp() call skips the auditing code, while putname()
doesn't.

mount_block_root allocates a PATH_MAX buffer via __getname_gfp, and then
calls get_fs_names to fill the buffer. That function can call
get_filesystem_list which assumes that that buffer is a full page in
size. On arches where PAGE_SIZE != 4k, then this could potentially
overrun.

In practice, it's hard to imagine the list of filesystem names even
approaching 4k, but it's best to be safe. Just allocate a page for this
purpose instead.

With this, we can also remove the __getname_gfp() definition since there
are no more callers.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2228,8 +2228,7 @@ extern void __init vfs_caches_init(unsigned long);
 
 extern struct kmem_cache *names_cachep;
 
-#define __getname_gfp(gfp)	kmem_cache_alloc(names_cachep, (gfp))
-#define __getname()		__getname_gfp(GFP_KERNEL)
+#define __getname()		kmem_cache_alloc(names_cachep, GFP_KERNEL)
 #define __putname(name)		kmem_cache_free(names_cachep, (void *)(name))
 #ifndef CONFIG_AUDITSYSCALL
 #define putname(name)   __putname(name)

init/do_mounts.c

@@ -353,8 +353,9 @@ static int __init do_mount_root(char *name, char *fs, int flags, void *data)
 
 void __init mount_block_root(char *name, int flags)
 {
-	char *fs_names = __getname_gfp(GFP_KERNEL
-		| __GFP_NOTRACK_FALSE_POSITIVE);
+	struct page *page = alloc_page(GFP_KERNEL |
+					__GFP_NOTRACK_FALSE_POSITIVE);
+	char *fs_names = page_address(page);
 	char *p;
 #ifdef CONFIG_BLOCK
 	char b[BDEVNAME_SIZE];
@@ -406,7 +407,7 @@ retry:
 #endif
 	panic("VFS: Unable to mount root fs on %s", b);
 out:
-	putname(fs_names);
+	put_page(page);
 }
  
 #ifdef CONFIG_ROOT_NFS