From 10b1fbdb0a0ca91847a534ad26d0bc250c25b74f Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 4 Nov 2006 13:03:00 -0800 Subject: [PATCH] Make sure "user->sigpending" count is in sync The previous commit (45c18b0bb579b5c1b89f8c99f1b6ffa4c586ba08, aka "Fix unlikely (but possible) race condition on task->user access") fixed a potential oops due to __sigqueue_alloc() getting its "user" pointer out of sync with switch_user(), and accessing a user pointer that had been de-allocated on another CPU. It still left another (much less serious) problem, where a concurrent __sigqueue_alloc and swich_user could cause sigqueue_alloc to do signal pending reference counting for a _different_ user than the one it then actually ended up using. No oops, but we'd end up with the wrong signal accounting. Another case of Oleg's eagle-eyes picking up the problem. This is trivially fixed by just making sure we load whichever "user" structure we decide to use (it doesn't matter _which_ one we pick, we just need to pick one) just once. Acked-by: Oleg Nesterov Cc: Andrew Morton Cc: Ingo Molnar Signed-off-by: Linus Torvalds --- kernel/signal.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index 7ed8d5304be..df18c167a2a 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -267,18 +267,25 @@ static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags, int override_rlimit) { struct sigqueue *q = NULL; + struct user_struct *user; - atomic_inc(&t->user->sigpending); + /* + * In order to avoid problems with "switch_user()", we want to make + * sure that the compiler doesn't re-load "t->user" + */ + user = t->user; + barrier(); + atomic_inc(&user->sigpending); if (override_rlimit || - atomic_read(&t->user->sigpending) <= + atomic_read(&user->sigpending) <= t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur) q = kmem_cache_alloc(sigqueue_cachep, flags); if (unlikely(q == NULL)) { - atomic_dec(&t->user->sigpending); + atomic_dec(&user->sigpending); } else { INIT_LIST_HEAD(&q->list); q->flags = 0; - q->user = get_uid(t->user); + q->user = get_uid(user); } return(q); }