On 07/07/2013 06:12 AM, Jeff King wrote:
> On Sat, Jul 06, 2013 at 09:48:52PM +0200, Michael Haggerty wrote:
>
>> When and if resolve_symlink() is called, then that function is
>> correctly told to treat the buffer as (PATH_MAX - 5) characters long.
>> This part is correct. However:
>>
>> * If LOCK_NODEREF was specified, then resolve_symlink() is never
>> called.
>>
>> * If resolve_symlink() is called but the path is not a symlink, then
>> the length check is never applied.
>>
>> So it is possible for a path with length (PATH_MAX - 5 <= len <
>> PATH_MAX) to make it through the checks. When ".lock" is strcat()ted
>> to such a path, the lock_file::filename buffer is overflowed.
>
> Thanks for posting this. I independently discovered this about a month
> ago while working on an unrelated series, and then let it languish
> unseen and forgotten at the base of that almost-done series.
>
> So definitely a problem, and my patch looked almost identical to
> yours. The only difference is:
>
>> static int lock_file(struct lock_file *lk, const char *path, int flags)
>> {
>> - if (strlen(path) >= sizeof(lk->filename))
>> - return -1;
>> - strcpy(lk->filename, path);
>> /*
>> * subtract 5 from size to make sure there's room for adding
>> * ".lock" for the lock file name
>> */
>> + if (strlen(path) >= sizeof(lk->filename)-5)
>> + return -1;
>> + strcpy(lk->filename, path);
>> if (!(flags & LOCK_NODEREF))
>> resolve_symlink(lk->filename, sizeof(lk->filename)-5);
>
> It might be worth consolidating the magic "-5" into a constant near the
> comment, like this:
>
> diff --git a/lockfile.c b/lockfile.c
> index c6fb77b..2aeb2bb 100644
> --- a/lockfile.c
> +++ b/lockfile.c
> @@ -124,15 +124,16 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
>
> static int lock_file(struct lock_file *lk, const char *path, int flags)
> {
> - if (strlen(path) >= sizeof(lk->filename))
> - return -1;
> - strcpy(lk->filename, path);
> /*
> * subtract 5 from size to make sure there's room for adding
> * ".lock" for the lock file name
> */
> + static const size_t max_path_len = sizeof(lk->filename) - 5;
> + if (strlen(path) >= max_path_len)
> + return -1;
> + strcpy(lk->filename, path);
> if (!(flags & LOCK_NODEREF))
> - resolve_symlink(lk->filename, sizeof(lk->filename)-5);
> + resolve_symlink(lk->filename, max_path_len);
> strcat(lk->filename, ".lock");
> lk->fd = open(lk->filename, O_RDWR | O_CREAT | O_EXCL, 0666);
> if (0 <= lk->fd) {
>
> But either way, the fix looks good to me.
Yes, the constant is an improvement and Peff's version is also fine with me.
Michael
--
Michael Haggerty
mhagger@xxxxxxxxxxxx
http://softwareswirl.blogspot.com/
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