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

[Eric Biggers]

On Wed, Mar 18, 2020 at 09:39:40AM -0700, Eric Biggers wrote:
> 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@xxxxxxxxxx>
> > > 
> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Reported-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > > Fixes: acaefc25d21f ("[PATCH] libfs: add simple attribute files")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > > ---
> > >  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.
> 

Andrew, can you consider taking this patch?  Al has been ignoring it, and this
seems like a fairly important fix.  This bug affects many (most?) debugfs files,
and it affects years old kernels too not just recent ones.

Thanks,

- Eric