Re: [PATCH v6] fs: clear file privilege bits when mmap writing

On Sun, Jan 10, 2016 at 10:51:52PM +0300, Konstantin Khlebnikov wrote:

On Sun, Jan 10, 2016 at 10:30 PM, Al Viro [off-list ref] wrote:

quoted

On Sun, Jan 10, 2016 at 06:48:32PM +0300, Konstantin Khlebnikov wrote:

quoted

I think this should be done in mmap/mprotect. Code in sys_mmap is trivial.

In sys_mprotect you can check file_needs_remove_privs() and VM_SHARED
under mmap_sem, then if needed grab reference to struct file from vma and
clear suid after unlocking mmap_sem.

Which vma?  mprotect(2) can cover more than one mapping...  You'd have to
play interesting games to collect the set of affected struct file; it
_might_ be doable (e.g. by using task_work_add() to have the damn thing
trigger on the way to userland), but it would require some care to avoid
hitting the same file more than once - it might, after all, be mmapped
in more than one process, so racing mprotect() would need to be taken
into account.  Hell knows - might be doable, but I'm not sure it'll be
any prettier.

Ok, I didn't thought about that. mprotect don't have to be atomic for whole
range -- we could drop mmap_sem, clear suid from one file and restart it
for next vma and so on.

Won't be fun.  Even aside of the user-visible behaviour changes, you'll have
a lot of new corner cases, starting with the fact that you can't hold onto
vma - virtual address is the best you can do and vma you find after regaining
mmap_sem might start at lower address than one where you are restarting;
getting the splitting-related logics right will be interesting, to put it
mildly.

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