On Tue, May 19, 2020 at 10:06:32AM -0500, Eric W. Biederman wrote: > Kees Cook <keescook@xxxxxxxxxxxx> writes: > > > Hi, > > > > While looking at the code paths for the proposed O_MAYEXEC flag, I saw > > some things that looked like they should be fixed up. > > > > exec: Change uselib(2) IS_SREG() failure to EACCES > > This just regularizes the return code on uselib(2). > > > > exec: Relocate S_ISREG() check > > This moves the S_ISREG() check even earlier than it was already. > > > > exec: Relocate path_noexec() check > > This adds the path_noexec() check to the same place as the > > S_ISREG() check. > > > > fs: Include FMODE_EXEC when converting flags to f_mode > > This seemed like an oversight, but I suspect there is some > > reason I couldn't find for why FMODE_EXEC doesn't get set in > > f_mode and just stays in f_flags. > > So I took a look at this series. > > I think the belt and suspenders approach of adding code in open and then > keeping it in exec and uselib is probably wrong. My sense of the > situation is a belt and suspenders approach is more likely to be > confusing and result in people making mistakes when maintaining the code > than to actually be helpful. This is why I added the comments in fs/exec.c's redundant checks. When I was originally testing this series, I had entirely removed the checks in fs/exec.c, but then had nightmares about some kind of future VFS paths that would somehow bypass do_open() and result in execve() working on noexec mounts, there by allowing for the introduction of a really nasty security bug. The S_ISREG test is demonstrably too late (as referenced in the series), and given the LSM hooks, I think the noexec check is too late as well. (This is especially true for the coming O_MAYEXEC series, which will absolutely need those tests earlier as well[1] -- the permission checking is then in the correct place: during open, not exec.) I think the only question is about leaving the redundant checks in fs/exec.c, which I think are a cheap way to retain a sense of robustness. -Kees [1] https://lore.kernel.org/lkml/202005142343.D580850@keescook/ -- Kees Cook
