libfs: fix infoleak in simple_attr_read()

commit a65cab7d7f05c2061a3e2490257d3086ff3202c6 upstream.

Reading from a debugfs file at a nonzero position, without first reading
at position 0, leaks uninitialized memory to userspace.

It's a bit tricky to do this, since lseek() and pread() aren't allowed
on these files, and write() doesn't update the position on them.  But
writing to them with splice() *does* update the position:

	#define _GNU_SOURCE 1
	#include <fcntl.h>
	#include <stdio.h>
	#include <unistd.h>
	int main()
	{
		int pipes[2], fd, n, i;
		char buf[32];

		pipe(pipes);
		write(pipes[1], "0", 1);
		fd = open("/sys/kernel/debug/fault_around_bytes", O_RDWR);
		splice(pipes[0], NULL, fd, NULL, 1, 0);
		n = read(fd, buf, sizeof(buf));
		for (i = 0; i < n; i++)
			printf("%02x", buf[i]);
		printf("\n");
	}

Output:
	5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a30

Fix the infoleak by making simple_attr_read() always fill
simple_attr::get_buf if it hasn't been filled yet.

Reported-by: syzbot+fcab69d1ada3e8d6f06b@syzkaller.appspotmail.com
Reported-by: Alexander Potapenko <glider@google.com>
Fixes: acaefc25d21f ("[PATCH] libfs: add simple attribute files")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200308023849.988264-1-ebiggers@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/libfs.c b/fs/libfs.c
index 1463b03..5fd9cc0 100644 (file)
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -821,7 +821,7 @@ int simple_attr_open(struct inode *inode, struct file *file,
 {
        struct simple_attr *attr;
 
-       attr = kmalloc(sizeof(*attr), GFP_KERNEL);
+       attr = kzalloc(sizeof(*attr), GFP_KERNEL);
        if (!attr)
                return -ENOMEM;
 
@@ -861,9 +861,11 @@ ssize_t simple_attr_read(struct file *file, char __user *buf,
        if (ret)
                return ret;
 
-       if (*ppos) {            /* continued read */
+       if (*ppos && attr->get_buf[0]) {
+               /* continued read */
                size = strlen(attr->get_buf);
-       } else {                /* first read */
+       } else {
+               /* first read */
                u64 val;
                ret = attr->get(attr->data, &val);
                if (ret)