libfs: fix infoleak in simple_attr_read()
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 c686bd9..3759fba 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -891,7 +891,7 @@ 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;
@@ -931,9 +931,11 @@ 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)