linux-kernel - Re: [syzbot] [btrfs?] KASAN: slab-out-of-bounds Read in getname

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Message-ID: <20240131184318.GQ31555@twin.jikos.cz>
Date: Wed, 31 Jan 2024 19:43:18 +0100
From: David Sterba <dsterba@...e.cz>
To: Edward Adam Davis <eadavis@...com>
Cc: dsterba@...e.cz, clm@...com, daniel@...earbox.net, dsterba@...e.com,
	john.fastabend@...il.com, josef@...icpanda.com,
	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, liujian56@...wei.com,
	syzbot+33f23b49ac24f986c9e8@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [btrfs?] KASAN: slab-out-of-bounds Read in
 getname_kernel (2)

On Tue, Jan 16, 2024 at 09:09:47AM +0800, Edward Adam Davis wrote:
> On Mon, 15 Jan 2024 20:08:25 +0100, David Sterba wrote:
> > > > If ioctl does not pass in the correct tgtdev_name string, oob will occur because
> > > > "\0" cannot be found.
> > > >
> > > > Reported-and-tested-by: syzbot+33f23b49ac24f986c9e8@...kaller.appspotmail.com
> > > > Signed-off-by: Edward Adam Davis <eadavis@...com>
> > > > ---
> > > >  fs/btrfs/dev-replace.c | 6 ++++--
> > > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> > > > index f9544fda38e9..e7e96e57f682 100644
> > > > --- a/fs/btrfs/dev-replace.c
> > > > +++ b/fs/btrfs/dev-replace.c
> > > > @@ -730,7 +730,7 @@ static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
> > > >  int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
> > > >  			    struct btrfs_ioctl_dev_replace_args *args)
> > > >  {
> > > > -	int ret;
> > > > +	int ret, len;
> > > >
> > > >  	switch (args->start.cont_reading_from_srcdev_mode) {
> > > >  	case BTRFS_IOCTL_DEV_REPLACE_CONT_READING_FROM_SRCDEV_MODE_ALWAYS:
> > > > @@ -740,8 +740,10 @@ int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
> > > >  		return -EINVAL;
> > > >  	}
> > > >
> > > > +	len = strnlen(args->start.tgtdev_name, BTRFS_DEVICE_PATH_NAME_MAX + 1);
> > > >  	if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
> > > > -	    args->start.tgtdev_name[0] == '\0')
> > > > +	    args->start.tgtdev_name[0] == '\0' ||
> > > > +	    len == BTRFS_DEVICE_PATH_NAME_MAX + 1)
> > >
> > > I think srcdev_name would have to be checked the same way, but instead
> > > of strnlen I'd do memchr(name, 0, BTRFS_DEVICE_PATH_NAME_MAX). The check
> > > for 0 in [0] is probably pointless, it's just a shortcut for an empty
> > > buffer. We expect a valid 0-terminated string, which could be an invalid
> > > path but that will be found out later when opening the block device.
> > 
> > Please let me know if you're going to send an updated fix. I'd like to
> > get this fixed to close the syzbot report but also want to give you the
> > credit for debugging and fix.
> > 
> > The preferred fix is something like that:
> > 
> > --- a/fs/btrfs/dev-replace.c
> > +++ b/fs/btrfs/dev-replace.c
> > @@ -741,6 +741,8 @@ int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
> >         if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
> >             args->start.tgtdev_name[0] == '\0')
> >                 return -EINVAL;
> > +       args->start.srcdev_name[BTRFS_PATH_NAME_MAX] = 0;
> > +       args->start.tgtdev_name[BTRFS_PATH_NAME_MAX] = 0;
> This is not correct,
> 1. The maximum length of tgtdev_name is BTRFS_DEVICE_PATH_NAME_MAX + 1
> 2. strnlen should be used to confirm the presence of \0 in tgtdev_name
> 3. Input values should not be subjectively updated

Regarding that point I agree it's not the best handling and could be
confusing. There are multiple instances of that in the ioctl callbacks
so the proper fix is to add a helper doing the validity check (either
strnlen or memchr) and then use it.

The pattern to look for is "vol_args->name[BTRFS_PATH_NAME_MAX] = '\0';"
in ioctl.c (at least).

Let me know if you'd want to implement that.