elf_set_personality()
Peter De Schrijver
pdeschrijver at nvidia.com
Mon Feb 27 10:03:00 EST 2012
On Mon, Feb 27, 2012 at 02:04:53PM +0100, Russell King - ARM Linux wrote:
> On Mon, Feb 27, 2012 at 02:36:12PM +0200, Peter De Schrijver wrote:
> > Commit ID 5e143436d04465c937c1a242808a99c46393af3e
> > (fix personality flag propagation across an exec) makes a child process
> > inherit a number of personality flags from its parent. This includes the
> > READ_IMPLIES_EXEC flag.
>
> So, READ_IMPLIES_EXEC will only be leaked if it's already set.
>
> > Unfortunately this causes problems when debugging
> > android applications using gdbserver. It appears that gdbserver on android
> > has an executable stack. This causes the kernel to set the READ_IMPLIES_EXEC
> > flag on the gdbserver process.
>
> Ok.
>
> > So the child android process will also get this flag.
>
> And that means that attempting to mmap() stuff will also get executable
> protections in addition.
>
> > As soon as the android tries to mmap a readonly android shmem segment
> > (using the ashmem driver), ashmem will return EPERM, because the segment has
> > been exposed as readonly without exec permissions and the kernel will modify
> > any readonly mmap request into read and execute.
>
> This sounds like a problem. If you have two applications trying to use
> the ashmem driver, one without READ_IMPLIES_EXEC and one with
> READ_IMPLIES_EXEC, then it seems that ashmem will prevent the
> READ_IMPLIES_EXEC one from using such regions. That sounds like a
> (different) bug to me.
>
Good point. I don't know anything about the design ideas behind ashmem though.
Can anyone from android comment on this?
> > Why does the READ_IMPLIES_EXEC flag need to be propagated? Would it be
> > acceptable to not propagate it to child processes?
>
> It shouldn't propagate. But I think it's uncovered a separate problem
Ok. Should I write a patch for this? Seems easy enough to also clear the
READ_IMPLIES_EXEC bit when copying the child personality from the parent?
> in the ashmem driver.
Cheers,
Peter.
More information about the linux-arm-kernel
mailing list