exec: fix unsigned loop counter wrap in transfer_args_to_stack()

In the Linux kernel, the following vulnerability has been resolved:

exec: fix unsigned loop counter wrap in transfer_args_to_stack()

The stop value is derived from bprm->p >> PAGE_SHIFT. The index variable
is an unsigned long. If bprm->p drops below PAGE_SIZE and stop becomes
zero the loop condition index >= stop is always true.

After the index == 0 iteration the decrement wraps to ULONG_MAX and
bprm->page[ULONG_MAX] reads sizeof(void *) bytes in front of the array.
The pointer has wrapped to -1. That garbage pointer is then passed to
kmap_local_page() and PAGE_SIZE bytes are copied from wherever that
lands into the stack of the process being created. And the loop doesn't
terminate either...

Getting there only requires bprm->p < PAGE_SIZE. On !MMU
bprm_set_stack_limit() and bprm_hit_stack_limit() are empty. So the only
constraint on how far bprm->p is pushed down is valid_arg_len(), i.e.
that each individual string still fits in what is left.

bprm->p starts at PAGE_SIZE * MAX_ARG_PAGES - sizeof(void *) so a
single argument or environment string of a little over 31 pages leaves
it in the first page:

Oops - load access fault [#1]
CPU: 0 UID: 0 PID: 1 Comm: victim Not tainted 7.2.0-rc4 #1
epc : __memcpy+0xd4/0xf8
ra : transfer_args_to_stack+0xaa/0xae
s4 : ffffffffffffffff s2 : 0000000000000000
a1 : ffffffdc98000000 a2 : 0000000000001000
status: 0000000a00001880 badaddr: ffffffdc98000000 cause: 0000000000000005
[<801a5324>] __memcpy+0xd4/0xf8
[<800d5f6a>] load_flat_binary+0x43a/0x65e
[<800a2de4>] bprm_execve+0x1d4/0x316
[<800a351a>] do_execveat_common+0x12e/0x138
[<800a3d44>] __riscv_sys_execve+0x38/0x4e
Kernel panic - not syncing: Fatal exception in interrupt

This is an arcane bug but we should still fix it.

Count down from MAX_ARG_PAGES so the loop ends when index reaches stop,
stop == 0 included. The iterations performed are unchanged for every
other value of stop.

Only CONFIG_MMU=n builds are affected, transfer_args_to_stack() is used
by binfmt_flat and binfmt_elf_fdpic on nommu only.

The loop predates git history. commit 7e7ec6a93434
("elf_fdpic_transfer_args_to_stack(): make it generic") only moved it
from binfmt_elf_fdpic.c into fs/exec.c and narrowed the copy to the used
part of the first page. The condition and the decrement are unchanged
from 2.6.12-rc2.