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Re: [libvirt] [PATCH] phyp: Fixing possible buffer overflow



 On 07/16/2010 03:04 AM, Eduardo Otubo wrote:
On 07/15/2010 11:01 PM, Laine Stump wrote:
On 07/15/2010 06:37 PM, Eduardo Otubo wrote:
The line src/phyp/phyp_driver.c:427 was crashing by buffer overflow
if the return of the command wasn't<=10. The highest number for a
LPAR ID is 256 per machine, no need to allocate 10 bytes for it. So,
adjusting the correct size (+1 byte for the '\n') and checking for
errors.

This analysis doesn't make sense to me. On the one hand, you say that
the function was crashing if the return from the command wasn't<= 10
bytes long (implying that it sometimes was longer), on the other hand,
you say that part of the solution is to change the code to only handle
4 bytes (you forgot about the trailing NULL there, BTW - should have
been 3), because you'll never get more than that.

I'm sorry. My bad here. The lssyscfg (with the correct parameters and greps) will return something like this:

5
4
6
7
10
1
143

These are the LPAR IDs. Each line cannot be bigger than 3 characters (+1 for '\n'). So, I just allocate 4 bytes to hold these values, one at a time and memset it with zeros. If there's an error, there will be a single line with the error message, like this one:

"HSCL8018 The managed system rico was not found."

IMO the best choice is to loop in the result and if I can't find a '\n' after 3 character, I return an error. My mistake was to check the entire string returned from exec. My bad, sorry.

I think this patch is much clearer on what the problem is.
(this new patch is corrupt and doesn't apply. Not sure how you sent it, but I'm guessing your mailer mangled it. It's usually better to only send patches with git send-email, or to attach the output of git format-patch to the mail, rather than pasting it inline)

This version is closer *in location* to avoiding the overflow, but it's still comparing the wrong index - it's looking at the index into the full array (ret, ie "i") rather than into the small copy that is used to convert (id_c, ie "j").

Also, it still allows writing past the end of the array (i == j == 4, so it can write to id_c[4]). Even allowing a write to id_c[3] would be a problem, since that would overwrite the terminating NULL, thus creating the possibility that virStrToLong_i could overrun the end of the array (or, if virStrToLong_i fails, the VIR_ERROR following it would try to print a string that is not NULL terminated - a *much* worse prospect).

Additionally the 2nd occurence of "memset(id_c, 0, 10)" inside the loop that I noted in my last mail has been left unchanged (and this memset will be done every time the function is called, even if there is no overflow of string length, so it is *guaranteed* the buffer will be overflowed).

Finally, in the case that the output of the exec is too long, this new patch will simply exit from the loop early and return success, rather than logging an error and returning failure.

These problems *could* be fixed by 1) changing "i <= 4" to "j < 3", 2) fixing the 2nd memset to clear 4 bytes instead of 10, and 3) turning the case of "j >= 3" into an error instead of just exiting the loop.

However... did you look at the counter-proposed patch in my previous email? I'm assuming that either you didn't notice it, or that you're trying to come up with a patch that makes the fewest changes possible while fixing the perceived bug. I think that patch would be a better idea for several reasons, and as long as we're fixing this function, we may as well fix it in the best way possible.

It takes advantage of the fact that virStrToLong_i will terminate on any non-numeric (not just NULL) to avoid the copy completely thus eliminating any built-in limits, and uses virStrToLong_i's returned "endptr" in place of counting characters itself. The resulting advantages are:

1) It is more "future proof". Maybe for now lssyscfg will not have any id's > 256, that may change in the future, and we have no way of knowing how long this code will last. For example, even if there are never more than 999 of them, maybe someone down the road will decide that these id's should be randomly generated for security reasons or something.

2) Because there is no fixed limit, there is no possibility of any "off by one" bugs.

3) The complicated character copying loop has been eliminated.

4) Also, error and success exit paths have been merged, meaning that resources are cleared in a single place, making it more difficult to forget to free resources on one exit path but not the other.

All of these make the function easier to verify as bug-free now, and will make future maintenance easier, since it will be both easier to understand, and more difficult to break (this current iteration of patches is ample proof that the current design makes it more difficult to catch bugs ;-)


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