On Fri, Mar 21, 2025 at 11:26:03PM -0700, Kees Cook wrote: > On Sat, Mar 22, 2025 at 01:00:08AM +0000, Al Viro wrote: > > On Fri, Mar 21, 2025 at 09:45:39AM +0100, Christian Brauner wrote: > > > > > Afaict, the only way this data race can happen is if we jump to the > > > cleanup label and then reset current->fs->in_exec. If the execve was > > > successful there's no one to race us with CLONE_FS obviously because we > > > took down all other threads. > > > > Not really. > > Yeah, you found it. Thank you! > > > 1) A enters check_unsafe_execve(), sets ->in_exec to 1 > > 2) B enters check_unsafe_execve(), sets ->in_exec to 1 > > With 3 threads A, B, and C already running, fs->users == 3, so steps (1) > and (2) happily pass. > > > 3) A calls exec_binprm(), fails (bad binary) > > 4) A clears ->in_exec > > 5) C calls clone(2) with CLONE_FS and spawns D - ->in_exec is 0 > > D's creation bumps fs->users == 4. > > > 6) B gets through exec_binprm(), kills A and C, but not D. > > 7) B clears ->in_exec, returns > > > > Result: B and D share ->fs, B runs suid binary. > > > > Had (5) happened prior to (2), (2) wouldn't have set ->in_exec; > > had (5) happened prior to (4), clone() would've failed; had > > (5) been delayed past (6), there wouldn't have been a thread > > to call clone(). > > > > But in the window between (4) and (6), clone() doesn't see > > execve() in progress and check_unsafe_execve() has already > > been done, so it hadn't seen the extra thread. > > > > IOW, it really is racy. It's a counter, not a flag. > > Yeah, I would agree. Totally untested patch: > > diff --git a/fs/exec.c b/fs/exec.c > index 506cd411f4ac..988b8621c079 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1632,7 +1632,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm) > if (p->fs->users > n_fs) > bprm->unsafe |= LSM_UNSAFE_SHARE; > else > - p->fs->in_exec = 1; > + refcount_inc(&p->fs->in_exec); > spin_unlock(&p->fs->lock); > } > > @@ -1862,7 +1862,7 @@ static int bprm_execve(struct linux_binprm *bprm) > > sched_mm_cid_after_execve(current); > /* execve succeeded */ > - current->fs->in_exec = 0; > + refcount_dec(¤t->fs->in_exec); > current->in_execve = 0; > rseq_execve(current); > user_events_execve(current); > @@ -1881,7 +1881,7 @@ static int bprm_execve(struct linux_binprm *bprm) > force_fatal_sig(SIGSEGV); > > sched_mm_cid_after_execve(current); > - current->fs->in_exec = 0; > + refcount_dec(¤t->fs->in_exec); > current->in_execve = 0; > > return retval; The bump is conditional and with this patch you may be issuing refcount_dec when you declined to refcount_inc. A special case where there are others to worry about and which proceeds with an exec without leaving in any indicators is imo sketchy. I would argue it would make the most sense to serialize these execs. Vast majority of programs are single-threaded when they exec with an unshared ->fs, so they don't need to bear any overhead nor complexity modulo a branch. For any fucky case you can park yourself waiting for any pending exec to finish.
