linux-kernel - [PATCH 5.6 18/23] libfs: fix infoleak in simple_attr

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Date:   Tue, 31 Mar 2020 10:59:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        syzbot+fcab69d1ada3e8d6f06b@...kaller.appspotmail.com,
        Alexander Potapenko <glider@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH 5.6 18/23] libfs: fix infoleak in simple_attr_read()

From: Eric Biggers <ebiggers@...gle.com>

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

---
 fs/libfs.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -891,7 +891,7 @@ int simple_attr_open(struct inode *inode
 {
 	struct simple_attr *attr;
 
-	attr = kmalloc(sizeof(*attr), GFP_KERNEL);
+	attr = kzalloc(sizeof(*attr), GFP_KERNEL);
 	if (!attr)
 		return -ENOMEM;
 
@@ -931,9 +931,11 @@ ssize_t simple_attr_read(struct file *fi
 	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)