[BusyBox 0004174]: handle_errors() buffer underflow
bugs at busybox.net
bugs at busybox.net
Thu Jul 17 08:45:19 UTC 2008
A NOTE has been added to this issue.
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http://busybox.net/bugs/view.php?id=4174
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Reported By: cristic
Assigned To: BusyBox
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Project: BusyBox
Issue ID: 4174
Category: Other
Reproducibility: always
Severity: minor
Priority: normal
Status: assigned
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Date Submitted: 07-16-2008 16:11 PDT
Last Modified: 07-17-2008 01:45 PDT
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Summary: handle_errors() buffer underflow
Description:
In handle_errors() (libbb/bb_strtonum.c), the line "if (endptr[-1] == '-')"
can lead to a buffer underflow, and the outcome of the branch will
usually
depend on uninitialized memory. Here is an example that triggers the
bug:
chmod a.b c
This leads to a call to bb_strtoul("a", NULL, 10), which calls
strtoul("a",
&endptr, 10), which will set endptr to point to the beginning of the
buffer
storing "a". Then endptr is passed to handle_errors() which is illegally
examining endptr[-1].
Looking at the code, I was confused about the comment on the "weird"
feature. Is that still needed? On my libc, calling strtol('-", &endptr,
10) sets endptr as expected to the beginning of the buffer storing "-", so
that particular if statement doesn't seem to be needed at all.
BTW, this bug can be hit by various other tools, here are some other
examples:
kill -l a
printf "% *" B
setuidgid a ""
Thanks,
Cristian
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----------------------------------------------------------------------
vda - 07-16-08 16:25
----------------------------------------------------------------------
Nope.
unsigned long FAST_FUNC bb_strtoul(const char *arg, char **endp, int
base)
{
unsigned long v;
char *endptr;
if (!isalnum(arg[0])) return ret_ERANGE(); <==========!!!
errno = 0;
v = strtoul(arg, &endptr, base);
return handle_errors(v, endp, endptr);
}
We definitely know that arg[0] is a digit. Therefore strtoul will not set
endptr to &arg[0], at the very worst, it will be &arg[1]. And thus
endptr[-1]
is valid (it references arg[0]).
----------------------------------------------------------------------
vda - 07-16-08 16:28
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> Is that still needed? On my libc, calling strtol("-", &endptr, 10) sets
endptr as expected to the beginning of the buffer storing "-"...
Yes. This is not a problem. The problem is, strtol returns 0 and does not
set errno. I refuse to believe that string "-" is a valid representation
of number 'zero', and I don't want busybox tools to accept that as a valid
numeric input.
----------------------------------------------------------------------
cristic - 07-16-08 17:17
----------------------------------------------------------------------
> if (!isalnum(arg[0])) return ret_ERANGE(); <==========!!!
> ...
> We definitely know that arg[0] is a digit. Therefore strtoul will not
set
> endptr to &arg[0], at the very worst, it will be &arg[1]. And thus
endptr[-1]
> is valid (it references arg[0]).
Sure, but isalnum() tests if the character is alphanumeric (digit _or_
letter), not if it's a digit. But I agree that if you replace isalnum
with isdigit you solve the problem; quite a short fix.
----------------------------------------------------------------------
vda - 07-17-08 01:45
----------------------------------------------------------------------
Oh. indeed, its isalnum! :(
replacing isalnum with isdigit with isdigit is not good - it will break
conversions to base 16.
I think the working fix would be swapping "if (endptr[0])" and "if
(endptr[-1] == '-')" checks in handle_errors(). If strtoul indeed stopped
on the very first char, if (endptr[0])... will trigger and in the if body
it checks for isalnum() and returns. "if (endptr[-1]..." won't be
executed.
Issue History
Date Modified Username Field Change
======================================================================
07-16-08 16:11 cristic New Issue
07-16-08 16:11 cristic Status new => assigned
07-16-08 16:11 cristic Assigned To => BusyBox
07-16-08 16:25 vda Note Added: 0009494
07-16-08 16:28 vda Note Added: 0009504
07-16-08 17:17 cristic Note Added: 0009514
07-16-08 17:22 cristic Issue Monitored: cristic
07-17-08 01:45 vda Note Added: 0009564
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