Re: [patch] libstdc++/67173 Fix filesystem::canonical for Solaris 10.

[Martin Sebor <msebor@gmail.com>]

On 09/17/2015 05:16 AM, Jonathan Wakely wrote:
> 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.

Yes, it is a corner case, and it's not really avoidable in the case
of hard links. For symlinks, POSIX defines the SYMLOOP_MAX constant
as the maximum, with the _SC_SYMLOOP_MAX and _PC_SYMLOOP_MAX
sysconf and pathconf variables. Otherwise 40 seems reasonable.

With this, I'll let you get back to work -- I think we've beat this
function to death ;)

Martin