Re: elf_set_personality()

[Peter De Schrijver <pdeschrijver@xxxxxxxxxx>]

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.

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