Re: [PATCH v2 4/7] fs/proc/task_mmu.c: shift mm_access() from m_start() to proc_maps_open()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Aug 05, 2014 at 09:46:55PM +0200, Oleg Nesterov wrote:
> A simple test-case from Kirill Shutemov
> 
> 	cat /proc/self/maps >/dev/null
> 	chmod +x /proc/self/net/packet
> 	exec /proc/self/net/packet
> 
> makes lockdep unhappy, cat/exec take seq_file->lock + cred_guard_mutex in
> the opposite order.
> 
> It's a false positive and probably we should not allow "chmod +x" on proc
> files. Still I think that we should avoid mm_access() and cred_guard_mutex
> in sys_read() paths, security checking should happen at open time. Besides,
> this doesn't even look right if the task changes its ->mm between m_stop()
> and m_start().
> 
> Add the new "mm_struct *mm" member into struct proc_maps_private and change
> proc_maps_open() to initialize it using proc_mem_open(). Change m_start() to
> use priv->mm if atomic_inc_not_zero(mm_users) succeeds or return NULL (eof)
> otherwise.
> 
> The only complication is that proc_maps_open() users should additionally do
> mmdrop() in fop->release(), add the new proc_map_release() helper for that.
> 
> Note: this is the user-visible change, if the task execs after open("maps")
> the new ->mm won't be visible via this file. I hope this is fine, and this
> matches /proc/pid/mem bahaviour.
> 
> Reported-by: "Kirill A. Shutemov" <kirill@shutemov.name>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
-- 
 Kirill A. Shutemov