[Bug libc/19728] New: out of bounds stack read in libidn function idna_to_ascii_4i

["hanno at hboeck dot de" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19728

            Bug ID: 19728
           Summary: out of bounds stack read in libidn function
                    idna_to_ascii_4i
           Product: glibc
           Version: 2.23
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: hanno at hboeck dot de
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 9038
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9038&action=edit
sample input

I had reported this a while ago to the developer of libidn, glibc bundles that
code. libidn has committed my patch here:
http://git.savannah.gnu.org/gitweb/?p=libidn.git;a=commit;h=f20ce1128fb7f4d33297eee307dddaf0f92ac72d

When passing an input of exactly 64 bytes to the idn tool it will
generate an out of bounds stack read.
This happens in the function idna_to_ascii_4i.

In Line 213 if the input is less than 64 bytes it will zero-terminate
the string. However if it's exactly 64 bytes the input will fill the
out buffer and no zero termination will happen. Therefore the strlen
call in line 271 will cause an out of bounds.

The strlen (out) > 63 check doesn't really make sense,
because inside a 64 byte buffer there can never be a correct
zero-terminated string longer than 63 bytes. Therefore I've removed
that check.

Found with the help of american fuzzy lop.

Address Sanitizer output:

==8591==ERROR: AddressSanitizer: stack-buffer-overflow on address
0x7ffd6c39b5e0 at pc 0x7eff8b780e00 bp 0x7ffd6c39b420 sp 0x7ffd6c39b3f0
READ of size 66 at 0x7ffd6c39b5e0 thread T0
    #0 0x7eff8b780dff in strlen
(/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x32dff)
    #1 0x412220 in idna_to_ascii_4i /mnt/ram/libidn-1.31-vanilla/lib/idna.c:269
    #2 0x4131fb in idna_to_ascii_4z /mnt/ram/libidn-1.31-vanilla/lib/idna.c:519
    #3 0x403582 in main /mnt/ram/libidn-1.31-vanilla/src/idn.c:374
    #4 0x7eff8b3d0f9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    #5 0x401c78 (/mnt/ram/libidn-1.31-vanilla/src/idn+0x401c78)

Address 0x7ffd6c39b5e0 is located in stack of thread T0 at offset 96 in frame
    #0 0x412c10 in idna_to_ascii_4z /mnt/ram/libidn-1.31-vanilla/lib/idna.c:472

  This frame has 1 object(s):
    [32, 96) 'buf' <== Memory access at offset 96 overflows this variable
HINT: this may be a false positive if your program uses some custom stack
unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow ??:0 strlen
Shadow bytes around the buggy address:
  0x10002d86b660: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10002d86b670: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10002d86b680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10002d86b690: 00 00 00 00 f1 f1 f1 f1 00 f4 f4 f4 00 00 00 00
  0x10002d86b6a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10002d86b6b0: f1 f1 f1 f1 00 00 00 00 00 00 00 00[f3]f3 f3 f3  
  0x10002d86b6c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10002d86b6d0: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10002d86b6e0: 00 f4 f4 f4 f2 f2 f2 f2 00 f4 f4 f4 f2 f2 f2 f2
  0x10002d86b6f0: 00 f4 f4 f4 f2 f2 f2 f2 00 f4 f4 f4 f2 f2 f2 f2
  0x10002d86b700: 00 f4 f4 f4 f2 f2 f2 f2 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe

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