On 11/08/2020 01:05, Al Viro wrote: > On Tue, Aug 11, 2020 at 12:43:52AM +0200, Mickaël Salaün wrote: > >> Hooking on open is a simple design that enables processes to check files >> they intend to open, before they open them. > > Which is a good thing, because...? > >> From an API point of view, >> this series extends openat2(2) with one simple flag: O_MAYEXEC. The >> enforcement is then subject to the system policy (e.g. mount points, >> file access rights, IMA, etc.). > > That's what "unspecified" means - as far as the kernel concerned, it's > "something completely opaque, will let these hooks to play, semantics is > entirely up to them". I see it as an access controls mechanism; access may be granted or denied, as for O_RDONLY, O_WRONLY or (non-Linux) O_EXEC. Even for common access controls, there are capabilities to bypass them (i.e. CAP_DAC_OVERRIDE), but multiple layers may enforce different complementary policies. > >> Checking on open enables to not open a file if it does not meet some >> requirements, the same way as if the path doesn't exist or (for whatever >> reasons, including execution permission) if access is denied. It is a >> good practice to check as soon as possible such properties, and it may >> enables to avoid (user space) time-of-check to time-of-use (TOCTOU) >> attacks (i.e. misuse of already open resources). > > ????? You explicitly assume a cooperating caller. As said in the below (removed) reply, no, quite the contrary. > If it can't be trusted > to issue the check between open and use, or can be manipulated (ptraced, > etc.) into not doing so, how can you rely upon the flag having been passed > in the first place? And TOCTOU window is definitely not wider that way. OK, I guess it would be considered a bug in the application (e.g. buggy resource management between threads). > > If you want to have it done immediately after open(), bloody well do it > immediately after open. If attacker has subverted your control flow to the > extent that allows them to hit descriptor table in the interval between > these two syscalls, you have already lost - they'll simply prevent that > flag from being passed. > > What's the point of burying it inside openat2()? A convenient multiplexor > to hook into? We already have one - it's called do_syscall_... > To check as soon as possible without opening something that should not be opened in the first place. Isn't a dedicated syscall a bit too much for this feature? What about adding a new command/flag to fcntl(2)?
