Re: [PATCH] libfs: fix infoleak in simple_attr_read()

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Mar 13, 2020 at 09:45:11AM -0700, Eric Biggers wrote:
> On Sat, Mar 07, 2020 at 06:38:49PM -0800, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > 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>
> > ---
> >  fs/libfs.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/libfs.c b/fs/libfs.c
> > index c686bd9caac6..3759fbacf522 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)
> > -- 
> > 2.25.1
> 
> Any comments on this?  Al, seems this is something you should pick up?
> 
> - Eric

Ping.