Re: vfs: oops on open_by_handle_at() in linux-next

[Al Viro]

On Sun, Oct 07, 2012 at 08:32:51PM -0700, Hugh Dickins wrote:
> Thank you, Sasha: this should fix it, and similar in other FSes.
> 
> 
> [PATCH] tmpfs,ceph,gfs2,isofs,reiserfs,xfs: fix fh_len checking
> 
> Fuzzing with trinity oopsed on the 1st instruction of shmem_fh_to_dentry(),
> 	u64 inum = fid->raw[2];
> which is unhelpfully reported as at the end of shmem_alloc_inode():
> 
> BUG: unable to handle kernel paging request at ffff880061cd3000
> IP: [<ffffffff812190d0>] shmem_alloc_inode+0x40/0x40
> Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> Call Trace:
>  [<ffffffff81488649>] ? exportfs_decode_fh+0x79/0x2d0
>  [<ffffffff812d77c3>] do_handle_open+0x163/0x2c0
>  [<ffffffff812d792c>] sys_open_by_handle_at+0xc/0x10
>  [<ffffffff83a5f3f8>] tracesys+0xe1/0xe6
> 
> Right, tmpfs is being stupid to access fid->raw[2] before validating that
> fh_len includes it: the buffer kmalloc'ed by do_sys_name_to_handle() may
> fall at the end of a page, and the next page not be present.
> 
> But some other filesystems (ceph, gfs2, isofs, reiserfs, xfs) are being
> careless about fh_len too, in fh_to_dentry() and/or fh_to_parent(), and
> could oops in the same way: add the missing fh_len checks to those.

TBH, I really don't like it.  How about putting minimal acceptable fhandle
length into export_operations instead?
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