A heap-buffer-overflow issue in revstr function in Misc.c:146

[wcventure]

Hi,

There exist a heap buffer-overflow issue in revstr function in Misc.c:146. Space of size is allocated for pointer p in p = (char *)calloc(size, sizeof(char));. However, the index used in for (i = size; i >= 0; i--, s++) is out of bound.

void revstr(char *str)
{
	char *p, *s;
	int i;
	int size;

	if ((size = strlen(str)) == 0)
		return;
	p = (char *)calloc(size, sizeof(char));
	s = p;
	for (i = size; i >= 0; i--, s++)
		*s = *(str + i - 1);
	*s = '\0';
	strncpy(str, p, size);
	str[size] = '\0';
	free(p);
}

To fix this bug, a patched version could be:

--- aget-master-new/Misc.c   2021-06-09 15:47:35.962171900 +0800
+++ aget-master/Misc.c  2021-06-09 15:48:13.950936000 +0800
@@ -140,9 +140,9 @@ void revstr(char *str)

        if ((size = strlen(str)) == 0)
                return;
-       p = (char *)calloc(size, sizeof(char));
+       p = (char *)calloc(size+1, sizeof(char));
        s = p;
-       for (i = size; i >= 0; i--, s++)
+       for (i = size; i > 0; i--, s++)
                *s = *(str + i - 1);
        *s = '\0';
        strncpy(str, p, size);

Here is the report from AddressSanitizer for this issue:

=================================================================
==5839==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000000017 at pc 0x55b0344b9c89 bp 0x7ffc1b493450 sp 0x7ffc1b493440
WRITE of size 1 at 0x602000000017 thread T0
    #0 0x55b0344b9c88 in revstr /Aget-master/Misc.c:146
    #1 0x55b0344b97de in parse_url /Aget-master/aget-devel/Misc.c:84
    #2 0x55b0344b718e in main /Aget-master/aget-devel/main.c:108
    #3 0x7f5f56cea0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #4 0x55b0344b6a4d in _start (/Aget-master/aget-devel/aget+0x4a4d)

0x602000000017 is located 0 bytes to the right of 7-byte region [0x602000000010,0x602000000017)
allocated by thread T0 here:
    #0 0x7f5f56fe5dc6 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10ddc6)
    #1 0x55b0344b9bf4 in revstr /Aget-master/aget-devel/Misc.c:143
    #2 0x55b0344b97de in parse_url /Aget-master/aget-devel/Misc.c:84
    #3 0x55b0344b718e in main /Aget-master/aget-devel/main.c:108
    #4 0x7f5f56cea0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

SUMMARY: AddressSanitizer: heap-buffer-overflow /Aget-master/aget-devel/Misc.c:146 in revstr
Shadow bytes around the buggy address:
  0x0c047fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c047fff8000: fa fa[07]fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
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
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==5839==ABORTING