Messages in this thread | | | Date | Sat, 01 Aug 2009 08:12:14 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH v2] execve: must clear current->clear_child_tid |
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Oleg Nesterov a écrit : > On 07/31, Andrew Morton wrote: >> On Fri, 31 Jul 2009 17:38:14 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote: >> >>> >>> On Sat, 1 Aug 2009, Oleg Nesterov wrote: >>>> Perhaps it is better to change mm_release() ? It has to play with >>>> ->clear_child_tid anyway. >>> Ahh. I take back my previous Ack. Your patch is better. I'll ack that >>> instead. >>> >> 'k, thanks. I shall compulsively watch my inbox awaiting the signed-off >> and tested version ;) > > I did some testing, but didn't try to check if this patches fixes the > origianal problem. It obviously should... Still I removed Tested-by tag. > But added Linus's ack, the patch is the same. > > ------------------------------------------------------------------------------ > [PATCH v2] execve: must clear current->clear_child_tid > > From: Eric Dumazet <eric.dumazet@gmail.com> > > While looking at Jens Rosenboom bug report > (http://lkml.org/lkml/2009/7/27/35) about strange sys_futex call done from > a dying "ps" program, we found following problem. > > clone() syscall has special support for TID of created threads. This > support includes two features. > > One (CLONE_CHILD_SETTID) is to set an integer into user memory with the > TID value. > > One (CLONE_CHILD_CLEARTID) is to clear this same integer once the created > thread dies. > > The integer location is a user provided pointer, provided at clone() > time. > > kernel keeps this pointer value into current->clear_child_tid. > > At execve() time, we should make sure kernel doesnt keep this user > provided pointer, as full user memory is replaced by a new one. > > As glibc fork() actually uses clone() syscall with CLONE_CHILD_SETTID and > CLONE_CHILD_CLEARTID set, chances are high that we might corrupt user > memory in forked processes. > > Following sequence could happen: > > 1) bash (or any program) starts a new process, by a fork() call that > glibc maps to a clone( ... CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID > ...) syscall > > 2) When new process starts, its current->clear_child_tid is set to a > location that has a meaning only in bash (or initial program) context > (&THREAD_SELF->tid) > > 3) This new process does the execve() syscall to start a new program. > current->clear_child_tid is left unchanged (a non NULL value) > > 4) If this new program creates some threads, and initial thread exits, > kernel will attempt to clear the integer pointed by > current->clear_child_tid from mm_release() : > > if (tsk->clear_child_tid > && !(tsk->flags & PF_SIGNALED) > && atomic_read(&mm->mm_users) > 1) { > u32 __user * tidptr = tsk->clear_child_tid; > tsk->clear_child_tid = NULL; > > /* > * We don't check the error code - if userspace has > * not set up a proper pointer then tough luck. > */ > << here >> put_user(0, tidptr); > sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0); > } > > 5) OR : if new program is not multi-threaded, but spied by /proc/pid > users (ps command for example), mm_users > 1, and the exiting program > could corrupt 4 bytes in a persistent memory area (shm or memory mapped > file) > > If current->clear_child_tid points to a writeable portion of memory of the > new program, kernel happily and silently corrupts 4 bytes of memory, with > unexpected effects. > > Fix is straightforward and should not break any sane program. > > Reported-by: Jens Rosenboom <jens@mcbone.net> > Acked-by: Linus Torvalds <torvalds@linux-foundation.org> > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> > Signed-off-by: Oleg Nesterov <oleg@redhat.com> > --- > > kernel/fork.c | 22 +++++++++++----------- > 1 file changed, 11 insertions(+), 11 deletions(-) > > --- WAIT/kernel/fork.c~CLEARTID 2009-07-02 19:27:36.000000000 +0200 > +++ WAIT/kernel/fork.c 2009-08-01 03:36:59.000000000 +0200 > @@ -568,18 +568,18 @@ void mm_release(struct task_struct *tsk, > * the value intact in a core dump, and to save the unnecessary > * trouble otherwise. Userland only wants this done for a sys_exit. > */ > - if (tsk->clear_child_tid > - && !(tsk->flags & PF_SIGNALED) > - && atomic_read(&mm->mm_users) > 1) { > - u32 __user * tidptr = tsk->clear_child_tid; > + if (tsk->clear_child_tid) { > + if (!(tsk->flags & PF_SIGNALED) && > + atomic_read(&mm->mm_users) > 1) { > + /* > + * We don't check the error code - if userspace has > + * not set up a proper pointer then tough luck. > + */ > + put_user(0, tsk->clear_child_tid); > + sys_futex(tsk->clear_child_tid, FUTEX_WAKE, > + 1, NULL, NULL, 0); > + } > tsk->clear_child_tid = NULL; > - > - /* > - * We don't check the error code - if userspace has > - * not set up a proper pointer then tough luck. > - */ > - put_user(0, tidptr); > - sys_futex(tidptr, FUTEX_WAKE, 1, NULL, NULL, 0); > } > } > >
Thanks Oleg, you are right this seems cleaner.
I only wonder about core dumping, since mm_release() is also used by exiting tasks.
Isnt clear_child_tid used by gdb or other debugger ? (The tid value is carefuly untouched in case of a core dump, but maybe gdb needs the current->clear_child_tid for whatever reason, for example to get TID address in user memory ?
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