buffer-underflow on _vsnprintf when len=0
Helica-core opened this issue · comments
at _vsnprintf() function, when len=0 and str is a valid buffer. it will write str[-1] with a zero
int _vsnprintf(char *str, size_t len, const char *fmt, va_list ap)
{
struct _output_args args;
int wlen;
args.outstr = str;
args.len = len;
args.pos = 0;
wlen = _printf_engine(&_vsnprintf_output, (void *)&args, fmt, ap);
if (args.pos >= len) { <---------- args.pos = 0, True
str[len - 1] = '\0'; <---------- buffer underflow str[-1] = '\0'
} else {
str[wlen] = '\0';
}
return wlen;
}
asan confirmed this issue:
─$ ./a.out
=================================================================
==54428==ERROR: AddressSanitizer: stack-buffer-underflow on address 0x7ffc8ab6ee0f at pc 0x5581bf4bdb06 bp 0x7ffc8ab6eba0 sp 0x7ffc8ab6eb98
WRITE of size 1 at 0x7ffc8ab6ee0f thread T0
#0 0x5581bf4bdb05 in _vsnprintf /mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/libc_printf.c:325
#1 0x5581bf4bd736 in _snprintf /mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/libc_printf.c:278
#2 0x5581bf4c1002 in main /mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/libc_printf.c:1017
#3 0x7f784140f189 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
#4 0x7f784140f244 in __libc_start_main_impl ../csu/libc-start.c:381
#5 0x5581bf4bd160 in _start (/mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/a.out+0x1160)
Address 0x7ffc8ab6ee0f is located in stack of thread T0 at offset 31 in frame
#0 0x5581bf4c0e4c in main /mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/libc_printf.c:1012
This frame has 1 object(s):
[32, 160) 's' (line 1013) <== Memory access at offset 31 underflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
(longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-underflow /mnt/hgfs/iotcpa/tmp/bootrom/sectest/printf/libc_printf.c:325 in _vsnprintf
Shadow bytes around the buggy address:
0x100011565d70: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
0x100011565d80: 00 00 00 f3 f3 f3 f3 f3 00 00 00 00 00 00 00 00
0x100011565d90: 00 00 00 00 f1 f1 f1 f1 00 00 00 f3 f3 f3 f3 f3
0x100011565da0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x100011565db0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
=>0x100011565dc0: f1[f1]00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x100011565dd0: 00 00 f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00
0x100011565de0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x100011565df0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x100011565e00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x100011565e10: 00 00 00 00 00 00 00 00 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
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
==54428==ABORTING