exec: fix unsigned loop counter wrap in transfer_args_to_stack()

Overview

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.

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < a9eb5c4949008034909bc34ecfa0843ecc1d0ab3
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 67cf5cdad823afb0530d6d0341fbf4ca07e93a09
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 66e20942890a383eb39b2009a2ceb4c2ebec37ef
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < c62bb00caba66e01fb578d5f0302f247dc64930a
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < dfc2a00742af4cb7251c1a8fbce4fbae3cc0de4e
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 2bc6bf70d41055377f390d06f0f3521deb62fd3b
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 55fa2c7f2b15583d1a2fe1b5abcc24377359339f
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 16cc4f5c1c4b9e45eca7f7deefa5410a292db599
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.