Re: [PATCH] New PT_GNU_COMPAT segment header extension
From: Andrew Morton
Date: Wed Feb 01 2012 - 17:13:16 EST
On Mon, 09 Jan 2012 06:54:19 -0500 (EST)
Ramon de C Valle <rcvalle@xxxxxxxxxx> wrote:
> These patches introduces the PT_GNU_COMPAT segment header to indicate the
> kernel whether an ELF binary or a shared library needs to have all readable
> virtual memory mappings also executable (i.e. READ_IMPLIES_EXEC personality)
> and if necessary, it allows more compatibility modes to be implemented in
> the future.
> Currently, an binary that needs only executable stack have unnecessarily all
> readable virtual memory mappings also executable. This is because the kernel
> does not know if the PF_X flag set in the PT_GNU_STACK segment header of the
> binary indicates it needs only stack executable or all readable virtual
> memory mappings also executable. Consequently, the kernel sets the
> READ_IMPLIES_EXEC personality upon loading any binary with the PT_GNU_STACK
> segment header and the PF_X flag set.
> Furthermore, if the PT_GNU_STACK segment header with the PT_X flag is set in
> any of the shared libraries a binary is linked, the kernel does not know if
> the PF_X flag set in the PT_GNU_STACK segment header of the shared library
> indicates it needs only stack executable or all readable virtual memory
> mappings also executable either. Consequently, GCC sets the PT_GNU_STACK
> segment header with the PF_X flag set upon compiling any binary linked to a
> shared library with the PT_GNU_STACK segment header and the PF_X flag set.
> This can result in applications unnoticeably having not only the stack, but
> also all readable virtual memory mappings also executable. These patches
> returns the original meaning and purpose of the PT_GNU_STACK segment header.
> >From the security standpoint, these binaries will rely upon lesser security
> extensions, such as heap consistency checking and others. Thus, improving
> the security of binaries that needs only the stack executable.
> These patches does not affect the current binaries that have the
> PT_GNU_STACK segment header, neither legacy binaries that most likely does
> not have this segment header either. For the existing binaries that
> necessarily need to have all readable virtual memory mappings also
> executable, the PT_GNU_STACK segment header can be easily converted to a
> PT_GNU_COMPAT segment header without recompilation.
> The following are important details about the changes made to the linker:
> * The PT_GNU_STACK and PT_GNU_COMPAT segment headers are mutually
> * The PT_GNU_STACK segment header has precedence over the PT_GNU_COMPAT
> segment header.
> * The PT_GNU_COMPAT segment header IS NOT created by default.
> * The PT_GNU_STACK segment header IS ALWAYS created by default--which is
> the original behaviour of the linker.
> These patches are completely non-intrusive. Over the time, once this
> compatibility mode is no longer needed, they can be removed with no
> subsequent effect.
<I have been poked>
I saw the patch but it's one of those ones which looks tricky/risky,
and is in an area with which I'm not sufficiently familiar. I used to
ask Roland McGrath to help out with this sort of thing, but he
dematerialised a while back. Perhaps Linus can give it some thought?
I'd suggest a resend: you've added useful info in later emails so there
would be benefit to bringing it all together in one place.
A couple of minor things:
- The patch will need a Signed-off-by:, as described in
- The term "compat" has a well-understood meaning in the kernel: it
refers to the support of 32-bit executables under 64-bit kernels.
Adding an unrelated PT_GNU_COMPAT muddies this. Can you think up a
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