Merge tag 'kallsyms_show_value-fix-v5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull sysfs module section fix from Kees Cook:
 "Fix sysfs module section output overflow.

  About a month after my kallsyms_show_value() refactoring landed, 0day
  noticed that there was a path through the kernfs binattr read handlers
  that did not have PAGE_SIZEd buffers, and the module "sections" read
  handler made a bad assumption about this, resulting in it stomping on
  memory when reached through small-sized splice() calls.

  I've added a set of tests to find these kinds of regressions more
  quickly in the future as well"

Sefltests-acked-by: Shuah Khan <skhan@linuxfoundation.org>

* tag 'kallsyms_show_value-fix-v5.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  selftests: splice: Check behavior of full and short splices
  module: Correctly truncate sysfs sections output

1 files changed, 19 insertions, 3 deletions

diff --git a/kernel/module.c b/kernel/module.c
index e7b4ff7e4fd0..8fa2600bde6a 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1520,18 +1520,34 @@ struct module_sect_attrs {
 	struct module_sect_attr attrs[];
 };
 
+#define MODULE_SECT_READ_SIZE (3 /* "0x", "\n" */ + (BITS_PER_LONG / 4))
 static ssize_t module_sect_read(struct file *file, struct kobject *kobj,
 				struct bin_attribute *battr,
 				char *buf, loff_t pos, size_t count)
 {
 	struct module_sect_attr *sattr =
 		container_of(battr, struct module_sect_attr, battr);
+	char bounce[MODULE_SECT_READ_SIZE + 1];
+	size_t wrote;
 
 	if (pos != 0)
 		return -EINVAL;
 
-	return sprintf(buf, "0x%px\n",
-		       kallsyms_show_value(file->f_cred) ? (void *)sattr->address : NULL);
+	/*
+	 * Since we're a binary read handler, we must account for the
+	 * trailing NUL byte that sprintf will write: if "buf" is
+	 * too small to hold the NUL, or the NUL is exactly the last
+	 * byte, the read will look like it got truncated by one byte.
+	 * Since there is no way to ask sprintf nicely to not write
+	 * the NUL, we have to use a bounce buffer.
+	 */
+	wrote = scnprintf(bounce, sizeof(bounce), "0x%px\n",
+			 kallsyms_show_value(file->f_cred)
+				? (void *)sattr->address : NULL);
+	count = min(count, wrote);
+	memcpy(buf, bounce, count);
+
+	return count;
 }
 
 static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
@@ -1580,7 +1596,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
 			goto out;
 		sect_attrs->nsections++;
 		sattr->battr.read = module_sect_read;
-		sattr->battr.size = 3 /* "0x", "\n" */ + (BITS_PER_LONG / 4);
+		sattr->battr.size = MODULE_SECT_READ_SIZE;
 		sattr->battr.attr.mode = 0400;
 		*(gattr++) = &(sattr++)->battr;
 	}