linux-kernel - Re: [PATCH][RESEND] PIE randomization

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Date:	Wed, 4 Jul 2007 04:25:54 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kratochvil <honza@...os.cz>, Ingo Molnar <mingo@...e.hu>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][RESEND] PIE randomization

On Wed, May 23, 2007 at 10:50:24AM +0200, Jiri Kosina wrote:
> From: Jan Kratochvil <honza@...os.cz>
> 
> This patch is using mmap()'s randomization functionality in such a way 
> that it maps the main executable of (specially compiled/linked -pie/-fpie) 
> ET_DYN binaries onto a random address (in cases in which mmap() is allowed 
> to perform a randomization).
> 
> Origin of this patch is in exec-shield
> (http://people.redhat.com/mingo/exec-shield/)
> 
> Signed-off-by: Jan Kratochvil <honza@...os.cz>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Roland McGrath <roland@...hat.com>
> Cc: Jakub Jelinek <jakub@...hat.com>

> -#define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE)
> +#define BAD_ADDR(x) ((unsigned long)(x) >= PAGE_MASK)
...
> @@ -442,8 +491,7 @@ static unsigned long load_elf_interp(str
>  			goto out_close;
>  	}
>  
> -	*interp_load_addr = load_addr;
> -	error = ((unsigned long)interp_elf_ex->e_entry) + load_addr;
> +	error = load_addr;
...
>  	if (elf_interpreter) {
> -		if (interpreter_type == INTERPRETER_AOUT)
> +		if (interpreter_type == INTERPRETER_AOUT) {
>  			elf_entry = load_aout_interp(&loc->interp_ex,
>  						     interpreter);
> -		else
> +		} else {
> +			unsigned long interp_map_addr;	/* unused */
> +
>  			elf_entry = load_elf_interp(&loc->interp_elf_ex,
>  						    interpreter,
> -						    &interp_load_addr);
> +						    &interp_map_addr,
> +						    load_bias);
> +			if (!BAD_ADDR(elf_entry)) {
> +				/*
> +				 * load_elf_interp() returns relocation
> +				 * adjustment
> +				 */
> +				interp_load_addr = elf_entry;
> +				elf_entry += loc->interp_elf_ex.e_entry;
> +			}
> +		}
>  		if (BAD_ADDR(elf_entry)) {
>  			force_sig(SIGSEGV, current);
>  			retval = IS_ERR((void *)elf_entry) ?

The above highlighted changes are the cause of random segfaults of PIE
binaries.  See
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=246623
The problem is if ld.so is prelinked to some address in the area where
the kernel actually maps it, particularly if elf_map in load_elf_interp
returns an address one page below its first PT_LOAD segments vaddr.
Then load_addr (it is a load bias actually) returned from load_elf_interp
is 0xfffff000 (on 32-bit kernels) and BAD_ADDR are all
addresses >= 0xfffff000 (on i?86).
The fix should be either changing the definition of BAD_ADDR to
e.g. IS_ERR_VALUE(x), or at least changing the if (!BAD_ADDR(elf_entry)) {
above to if (!IS_ERR_VALUE(elf_entry)) {, the second BAD_ADDR can already
stay, because at that place elf_entry is no longer a bias (difference
between actual and preferred load address), but an actual address, where
very high addresses are of course invalid.

	Jakub
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