Re: [PATCH] get_user_pages(..., write==1, ...) may return with readable pte.

[Nick Piggin <npiggin@suse.de>]

On Fri, Oct 13, 2006 at 03:33:42PM -0500, Robin Holt wrote:
> Handle the case in get_user_pages() when a call to __handle_mm_fault()
> inserts a writable pte, and a process doing dup_mmap converts it
> to readable before get_user_pages() does the subsequent request to
> follow_page().
> 
> 
> Signed-off-by: Robin Holt <holt@sgi.com>
> 
> ---
> 
> Hugh, Nick, and Linus,
> 
> I think I have tripped over another flavor of a get_user_pages bug
> we addressed back in 2005.  I do not have a test case to prove it is
> the issue I am trying to address, but I have done as thorough a code
> walk-through as I can.
> 
> Assume a pte is currently empty.  A first pthread is in the kernel on
> a call path which is leading to get_user_pages.  A second pthread is
> in the process of doing a fork.  The process doing get_user_pages()
> gets into __handle_mm_fault() and grabs ptl just before the process
> doing a fork attempts to grab the ptl to convert the pages to COW.
> __handle_mm_fault() will insert the writable pte and unlock ptl then
> return with VM_FAULT_WRITE set.  The process doing a fork then gets
> the lock and starts converting the pte to RO/COW.  The get_user_pages()
> process then clears FOLL_WRITE from foll_flags and calls follow_page()
> without write, adds to the map count for the page, but does not have a
> writable mapping.

Hi Robin,

dup_mmap holds mmap_sem for write. get_user_pages caller must hold it
for read.

So it think it is OK? But if not, then you can't just get rid of this
FOLL_WRITE bit, because then we get infinite loops when a 'force'
write access (eg. ptrace setting a breakpoint in text).