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SubjectRe: [PATCH][RESEND] PIE randomization
On Wed, May 23, 2007 at 10:50:24AM +0200, Jiri Kosina wrote:
> From: Jan Kratochvil <honza@jikos.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@jikos.cz>
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Roland McGrath <roland@redhat.com>
> Cc: Jakub Jelinek <jakub@redhat.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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