Re: [PATCH 1/2] module: Correctly truncate sysfs sections output

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+++ Kees Cook [06/08/20 23:35 -0700]:
The only-root-readable /sys/module/$module/sections/$section files
did not truncate their output to the available buffer size. While most
paths into the kernfs read handlers end up using PAGE_SIZE buffers,
it's possible to get there through other paths (e.g. splice, sendfile).
Actually limit the output to the "count" passed into the read function,
and report it back correctly. *sigh*

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/20200805002015.GE23458@shao2-debian
Fixes: ed66f991bb19 ("module: Refactor section attr into bin attribute")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Jessica Yu <jeyu@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Oof, thanks for fixing this!

Acked-by: Jessica Yu <jeyu@xxxxxxxxxx>

---
kernel/module.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index aa183c9ac0a2..08c46084d8cc 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;
	}
--
2.25.1




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