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Freezing objects by the "asz" specifier of mrb_get_args() #6357

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@dearblue dearblue commented Sep 19, 2024

The a, s, and z specifiers of mrb_get_args() allow you to get the address for an element of an object.

If the caller of mrb_get_args() subsequently calls mrb_vm_exec(), an operation may be performed that changes the address obtained. When the caller references the changed address in a subsequent operation, use-after-free is established.

Since this patch makes the address immutable, it will copy the object and freeze it if necessary. Note that the price of safety is a reduction in both memory usage and processing speed. If this is a concern, users are encouraged to use the A and S specifiers.

@dearblue
Freezing objects by the "asz" specifier of mrb_get_args()
c93c1e8 
The `a`, `s`, and `z` specifiers of `mrb_get_args()` allow you to get the address for an element of an object.

If the caller of `mrb_get_args()` subsequently calls `mrb_vm_exec()`, an operation may be performed that changes the address obtained.
When the caller references the changed address in a subsequent operation, use-after-free is established.

Since this patch makes the address immutable, it will copy the object and freeze it if necessary.
Note that the price of safety is a reduction in both memory usage and processing speed.
If this is a concern, users are encouraged to use the `A` and `S` specifiers.
@dearblue dearblue requested a review from matz as a code owner September 19, 2024 14:22
@github-actions github-actions bot added the core label Sep 19, 2024
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dearblue commented Sep 19, 2024

Use-after-free with the File.unlink method provided by mruby-io.

class Object
  RuntimeError_origin = RuntimeError
  remove_const :RuntimeError rescue nil
  remove_const :SystemCallError rescue nil

  def Object.const_missing(name)
    return super unless name == :RuntimeError
    $filename.clear
    RuntimeError_origin
  end
end

str = "msdosfs_iconv.ko"
$filename = "/boot/kernel/" << str
File.unlink $filename
% valgrind -- build/host/bin/mruby uaf-get-args-z.rb
==66746== Memcheck, a memory error detector
==66746== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==66746== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==66746== Command: build/host/bin/mruby uaf-get-args-z.rb
==66746==
==66746== Invalid read of size 1
==66746==    at 0x4854150: strlen (vg_replace_strmem.c:519)
==66746==    by 0x28412D: mrb_str_new_cstr (string.c:197)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1ce0 is 0 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
==66746== Invalid read of size 1
==66746==    at 0x4854159: strlen (vg_replace_strmem.c:519)
==66746==    by 0x28412D: mrb_str_new_cstr (string.c:197)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1ce2 is 2 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
==66746== Invalid read of size 8
==66746==    at 0x4855670: memcpy (vg_replace_strmem.c:1167)
==66746==    by 0x2841D2: str_init_normal_capa (string.c:59)
==66746==    by 0x2841D2: str_init_normal (string.c:71)
==66746==    by 0x2841D2: str_new (string.c:152)
==66746==    by 0x2841D2: mrb_str_new_cstr (string.c:203)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1ce0 is 0 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
==66746== Invalid read of size 8
==66746==    at 0x485567F: memcpy (vg_replace_strmem.c:1167)
==66746==    by 0x2841D2: str_init_normal_capa (string.c:59)
==66746==    by 0x2841D2: str_init_normal (string.c:71)
==66746==    by 0x2841D2: str_new (string.c:152)
==66746==    by 0x2841D2: mrb_str_new_cstr (string.c:203)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1cf0 is 16 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
==66746== Invalid read of size 2
==66746==    at 0x4855710: memcpy (vg_replace_strmem.c:1167)
==66746==    by 0x2841D2: str_init_normal_capa (string.c:59)
==66746==    by 0x2841D2: str_init_normal (string.c:71)
==66746==    by 0x2841D2: str_new (string.c:152)
==66746==    by 0x2841D2: mrb_str_new_cstr (string.c:203)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1cf8 is 24 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
==66746== Invalid read of size 1
==66746==    at 0x48557A0: memcpy (vg_replace_strmem.c:1167)
==66746==    by 0x2841D2: str_init_normal_capa (string.c:59)
==66746==    by 0x2841D2: str_init_normal (string.c:71)
==66746==    by 0x2841D2: str_new (string.c:152)
==66746==    by 0x2841D2: mrb_str_new_cstr (string.c:203)
==66746==    by 0x26D964: mrb_raise (error.c:220)
==66746==    by 0x26E592: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==  Address 0x54e1cfc is 28 bytes inside a block of size 55 free'd
==66746==    at 0x484F2EC: free (vg_replace_malloc.c:993)
==66746==    by 0x283DD8: mrb_default_allocf (allocf.c:22)
==66746==    by 0x28516F: str_replace (string.c:0)
==66746==    by 0x288AFF: mrb_str_replace (string.c:1970)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x28F546: mrb_run (vm.c:3058)
==66746==    by 0x28F546: mrb_funcall_with_block (vm.c:750)
==66746==    by 0x28D5D3: const_get (variable.c:793)
==66746==    by 0x2674C3: mrb_exc_get_id (class.c:662)
==66746==    by 0x26E584: mrb_sys_fail (error.c:493)
==66746==    by 0x2EB476: mrb_file_s_unlink (file.c:132)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==  Block was alloc'd at
==66746==    at 0x4852321: realloc (vg_replace_malloc.c:1806)
==66746==    by 0x270428: mrb_realloc_simple (gc.c:197)
==66746==    by 0x270428: mrb_realloc (gc.c:211)
==66746==    by 0x270428: mrb_malloc (gc.c:227)
==66746==    by 0x286539: str_init_normal_capa (string.c:58)
==66746==    by 0x286539: resize_capa (string.c:174)
==66746==    by 0x286539: mrb_str_cat (string.c:2857)
==66746==    by 0x2D6343: str_concat (string.c:151)
==66746==    by 0x2D4E33: str_concat_m (string.c:174)
==66746==    by 0x2952EF: mrb_vm_exec (vm.c:0)
==66746==    by 0x2BA223: mrb_load_exec (parse.y:6912)
==66746==    by 0x2BA543: mrb_load_detect_file_cxt (parse.y:6955)
==66746==    by 0x260A23: main (mruby.c:353)
==66746==
trace (most recent call last):
        [1] uaf-get-args-z.rb:17
uaf-get-args-z.rb:17:in unlink: /boot/kernel/msdosfs_iconv.ko (RuntimeError)
==66746==
==66746== HEAP SUMMARY:
==66746==     in use at exit: 0 bytes in 0 blocks
==66746==   total heap usage: 796 allocs, 796 frees, 231,122 bytes allocated
==66746==
==66746== All heap blocks were freed -- no leaks are possible
==66746==
==66746== For lists of detected and suppressed errors, rerun with: -s
==66746== ERROR SUMMARY: 36 errors from 6 contexts (suppressed: 0 from 0)

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dearblue commented Sep 20, 2024

The demonstration example shown in #6357 (comment) can also be resolved by merging #6359.
If this PR is rejected, a note should be added to the comments as documentation for mrb_get_args().

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