Re: [Bug 11063] New: lack of GNU_STACK header doesn't result in rwx stack on i386

Re: [Bug 11063] New: lack of GNU_STACK header doesn't result in rwx stack on i386

[Andrew Morton]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Wed,  9 Jul 2008 12:13:05 -0700 (PDT) bugme-daemon@bugzilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=11063
> 
>            Summary: lack of GNU_STACK header doesn't result in rwx stack on
>                     i386
>            Product: Memory Management
>            Version: 2.5
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: Other
>         AssignedTo: akpm@osdl.org
>         ReportedBy: pageexec@freemail.hu
> 
> 
> Latest working kernel version: unknown, probably before the introduction of
> unlimited argv
> Earliest failing kernel version: unknown, probably after the introduction of
> unlimited argv, 2.6.26-rc9 definitely fails
> Distribution:
> Hardware Environment:
> Software Environment:
> Problem Description:
> ELF/i386 traditionally has an executable stack which in the PT_GNU_STACK world
> is provided two ways: if the ELF has a RWE PT_GNU_STACK program header or if it
> lacks that program header. due to this regression, the latter case results in a
> non-executable stack with corresponding breakage of (old) userland binaries
> that use nested function trampolines.
> 
> this bug was probably introduced when the unlimited argv feature was merged: in
> that system the stack vma (and its vm_flags) is initialized too early, before
> the incoming process' ELF header is consulted, which on i386 means that the
> personality used for determining the executable status of the stack is that of
> the old process, not the incoming one. later, in setup_arg_pages, only the
> explicit setting (disable/enable) of stack executability is honoured,
> EXSTACK_DEFAULT isn't consulted (i guess the assumption was that the initial
> ->vm_flags setting got it right).
> 
> Steps to reproduce: run any ELF that lacks a PT_GNU_STACK program header and
> check its stack vma, it'll be rw-, instead of rwx.
> 

[Bug 11063][PATCH] exec: fix stack excutability without PT_GNU_STACK

[Hugh Dickins]

Kernel Bugzilla #11063 points out that on some architectures (e.g. x86_32)
exec'ing an ELF without a PT_GNU_STACK program header should default to an
executable stack; but this got broken by the unlimited argv feature because
stack vma is now created before the right personality has been established:
so breaking old binaries using nested function trampolines.

Therefore re-evaluate VM_STACK_FLAGS in setup_arg_pages, where stack
vm_flags used to be set, before the mprotect_fixup.  Checking through
our existing VM_flags, none would have changed since insert_vm_struct:
so this seems safer than finding a way through the personality labyrinth.

Reported-by: pageexec@freemail.hu
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: stable@kernel.org
---

 fs/exec.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.26-rc9/fs/exec.c	2008-06-21 08:41:19.000000000 +0100
+++ linux/fs/exec.c	2008-07-10 20:02:25.000000000 +0100
@@ -610,7 +610,7 @@ int setup_arg_pages(struct linux_binprm 
 	bprm->exec -= stack_shift;
 
 	down_write(&mm->mmap_sem);
-	vm_flags = vma->vm_flags;
+	vm_flags = VM_STACK_FLAGS;
 
 	/*
 	 * Adjust stack execute permissions; explicitly enable for

Re: [Bug 11063][PATCH] exec: fix stack excutability without PT_GNU_STACK

[pageexec]

On 10 Jul 2008 at 21:19, Hugh Dickins wrote:

> Kernel Bugzilla #11063 points out that on some architectures (e.g. x86_32)
> exec'ing an ELF without a PT_GNU_STACK program header should default to an
> executable stack; but this got broken by the unlimited argv feature because
> stack vma is now created before the right personality has been established:
> so breaking old binaries using nested function trampolines.
> 
> Therefore re-evaluate VM_STACK_FLAGS in setup_arg_pages, where stack
> vm_flags used to be set, before the mprotect_fixup.  Checking through
> our existing VM_flags, none would have changed since insert_vm_struct:
> so this seems safer than finding a way through the personality labyrinth.

alternatively, if there's a concern of stack_vma->vm_flags manipulation
during execve (maybe not now, but in the future or in non-ELF formats
that also want to rely on personality bits), you could opt for a safer

	vm_flags = vma->vm_flags | (VM_STACK_FLAGS & (VM_EXEC | VM_MAYEXEC));

to just recompute the exec rights related bits.

> Reported-by: pageexec@freemail.hu
> Signed-off-by: Hugh Dickins <hugh@veritas.com>
> Cc: stable@kernel.org
> ---
> 
>  fs/exec.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 2.6.26-rc9/fs/exec.c	2008-06-21 08:41:19.000000000 +0100
> +++ linux/fs/exec.c	2008-07-10 20:02:25.000000000 +0100
> @@ -610,7 +610,7 @@ int setup_arg_pages(struct linux_binprm 
>  	bprm->exec -= stack_shift;
>  
>  	down_write(&mm->mmap_sem);
> -	vm_flags = vma->vm_flags;
> +	vm_flags = VM_STACK_FLAGS;
>  
>  	/*
>  	 * Adjust stack execute permissions; explicitly enable for
> 

Re: [Bug 11063][PATCH] exec: fix stack excutability without PT_GNU_STACK

[Hugh Dickins]

On Thu, 10 Jul 2008, pageexec@freemail.hu wrote:
> On 10 Jul 2008 at 21:19, Hugh Dickins wrote:
> 
> > Therefore re-evaluate VM_STACK_FLAGS in setup_arg_pages, where stack
> > vm_flags used to be set, before the mprotect_fixup.  Checking through
> > our existing VM_flags, none would have changed since insert_vm_struct:
> > so this seems safer than finding a way through the personality labyrinth.
> 
> alternatively, if there's a concern of stack_vma->vm_flags manipulation
> during execve (maybe not now, but in the future or in non-ELF formats
> that also want to rely on personality bits), you could opt for a safer
> 
> 	vm_flags = vma->vm_flags | (VM_STACK_FLAGS & (VM_EXEC | VM_MAYEXEC));
> 
> to just recompute the exec rights related bits.

True.  It was a concern that crossed my mind (I was thinking particularly
of the VM_ACCOUNT flag, which gets added in once we deal with a writable
private mapping, but is set from the start here anyway), but I don't
think it's worth changing my

> > -	vm_flags = vma->vm_flags;
> > +	vm_flags = VM_STACK_FLAGS;

now that's already there in Linus' tree.  If a VM_flag gets added that
changes the picture, it might even need your line above to be changed.

Hugh