Re: fs: use-after-free in path_lookupat

[Dmitry Vyukov <dvyukov@google.com>]

On Sun, Mar 5, 2017 at 8:18 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sun, Mar 05, 2017 at 06:33:18PM +0100, Dmitry Vyukov wrote:
>
>> Added more debug output.
>>
>> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300",
>> &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0,
>> 0x1000)
>>
>> actually passes name="" because of the overlapping addresses. Flags
>> contain AT_EMPTY_PATH.
>
> Bloody hell...  So you end up with name == (char *)&handle->handle_type + 3?
> Looks like it would be a lot more useful to dump the actual contents of
> those suckers right before the syscall...
>
> Anyway, that explains WTF is going on.  The bug is in path_init() and
> it triggers when you pass something with dentry allocated by d_alloc_pseudo()
> as dfd, combined with empty pathname.  You need to have the file closed
> by another thread, and have that another thread get out of closing syscall
> (close(), dup2(), etc.) before the caller of path_init() gets to
> complete_walk().  We need to make sure that this sucker gets DCACHE_RCUPDATE
> while it's still guaranteed to be pinned down.  Could you try to reproduce
> with the patch below applied?
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 6f7d96368734..70840281a41c 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2226,11 +2226,16 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>                 nd->path = f.file->f_path;
>                 if (flags & LOOKUP_RCU) {
>                         rcu_read_lock();
> -                       nd->inode = nd->path.dentry->d_inode;
> -                       nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
> +                       if (unlikely(!(dentry->d_flags & DCACHE_RCUACCESS))) {
> +                               spin_lock(&dentry->d_lock);
> +                               dentry->d_flags |= DCACHE_RCUACCESS;
> +                               spin_unlock(&dentry->d_lock);
> +                       }
> +                       nd->inode = dentry->d_inode;
> +                       nd->seq = read_seqcount_begin(&dentry->d_seq);
>                 } else {
>                         path_get(&nd->path);
> -                       nd->inode = nd->path.dentry->d_inode;
> +                       nd->inode = dentry->d_inode;
>                 }
>                 fdput(f);
>                 return s;


Al, please send this patch officially. I am running with it since then
and have not seen the crashes, nor any other issues that look related.

Thanks!