Messages in this thread Patch in this message |  | | Date | Fri, 31 Jul 2009 12:00:03 +0200 | From | Eric Dumazet <> | Subject | [ PATCH] execve: must clear current->clear_child_tid |
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While looking at Jens Rosenboom bug report 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> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Tested-by: Jens Rosenboom <jens@mcbone.net> --- fs/compat.c | 1 + fs/exec.c | 1 + 2 files changed, 2 insertions(+)
diff --git a/fs/compat.c b/fs/compat.c index 94502da..deb1049 100644 --- a/fs/compat.c +++ b/fs/compat.c @@ -1550,6 +1550,7 @@ int compat_do_execve(char * filename, mutex_unlock(¤t->cred_guard_mutex); acct_update_integrals(current); free_bprm(bprm); + current->clear_child_tid = NULL; if (displaced) put_files_struct(displaced); return retval; diff --git a/fs/exec.c b/fs/exec.c index 4a8849e..e275652 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1343,6 +1343,7 @@ int do_execve(char * filename, mutex_unlock(¤t->cred_guard_mutex); acct_update_integrals(current); free_bprm(bprm); + current->clear_child_tid = NULL; if (displaced) put_files_struct(displaced); return retval;
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