On 16/09/15 17:42 -0600, Martin Sebor wrote:
I see now the first exists test will detect symlink loops in the original path. But I'm not convinced there isn't a corner case that's subject to a TOCTOU race condition between the first exists test and the while loop during which a symlink loop can be introduced.Suppose we call the function with /foo/bar as an argument and the path exists and contains no symlinks. result is / and cmpts is set to { foo, bar }. Just as the loop is entered, /foo/bar is replaced with a symlink containing /foo/bar. The loop then proceeds like so: 1. The first iteration removes foo from cmpts and sets result to /foo. cmpts is { bar }. 2. The second iteration removes bar from cmpts, sets result to /foo/bar, determines it's a symlink, reads its contents, sees it's an absolute pathname and replaces result with /. It then inserts the symlink's components { foo, bar } into cmpts. cmpts becomes { foo, bar }. exists(result) succeeds. 3. The next iteration of the loop has the same initial state as the first. But I could have very easily missed something that takes care of this corner case. If I did, sorry for the false alarm!
No, you're right. The TS says such filesystem races are undefined: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4099.html#fs.race.behavior but it would be nice to fail gracefully rather than DOS the application. The simplest approach would be to increment a counter every time we follow a symlink, and if it reaches some limit decide something is wrong and fail with ELOOP. I don't see how anything else can be 100% bulletproof, because a truly evil attacker could just keep altering the contents of symlinks so we keep ping-ponging between two or more paths. If we keep track of paths we've seen before the attacker could just keep changing the contents to a unique path each time, that initially exists as a file, but by the time we get to is_symlink() its become a symlink to a new path. So if we use a counter, what's a sane maximum? Is MAXSYMLINKS in <sys/param.h> the value the kernel uses? 20 seems quite low, I was thinking of a much higher number.
