Re: [PATCH] futex: futex_find_get_task remove credentails check

From: Darren Hart
Date: Wed Jun 30 2010 - 12:43:49 EST

On 06/30/2010 02:55 AM, Michal Hocko wrote:
On Wed 30-06-10 09:01:15, Michal Hocko wrote:
On Tue 29-06-10 09:41:02, Linus Torvalds wrote:
On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko<mhocko@xxxxxxx> wrote:

futex_find_get_task is currently used (through lookup_pi_state) from two
contexts, futex_requeue and futex_lock_pi_atomic. While credentials check
makes sense in the first code path, the second one is more problematic
because this check requires that the PI lock holder (pid parameter) has
the same uid and euid as the process's euid which is trying to lock the
same futex (current).

So exactly why does it make sense to check the credentials in the
first code path then?

I though that requeue needs this for security reasons (don't let requeue
process for other user), but when I thought about that again you are
right and the only what matters should be accessibility of the shared

And here is the patch which does the thing.


From 082c5ad2c482a8e78b61b17e213e750b006176aa Mon Sep 17 00:00:00 2001
From: Michal Hocko<mhocko@xxxxxxx>
Date: Wed, 30 Jun 2010 09:51:19 +0200
Subject: [PATCH] futex: futex_find_get_task remove credentails check

futex_find_get_task is currently used (through lookup_pi_state) from two
contexts, futex_requeue and futex_lock_pi_atomic. None of the paths
looks it needs the credentials check, though. Different (e)uids
shouldn't matter at all because the only thing that is important for
shared futex is the accessibility of the shared memory.

The credentail check results in glibc assert failure or process hang (if
glibc is compiled without assert support) for shared robust pthread
mutex with priority inheritance if a process tries to lock already held
lock owned by a process with a different euid:

pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.

The problem is that futex_lock_pi_atomic which is called when we try to
lock already held lock checks the current holder (tid is stored in the
futex value) to get the PI state. It uses lookup_pi_state which in turn
gets task struct from futex_find_get_task. ESRCH is returned either when
the task is not found or if credentials check fails.
futex_lock_pi_atomic simply returns if it gets ESRCH. glibc code,
however, doesn't expect that robust lock returns with ESRCH because it
should get either success or owner died.

Signed-off-by: Michal Hocko<mhocko@xxxxxxx>

Without hearing back from Ingo on the original intent of the credentials check, this looks right to me.

Acked-by: Darren Hart <dvhltc@xxxxxxxxxx>

kernel/futex.c | 17 ++++-------------
1 files changed, 4 insertions(+), 13 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index e7a35f1..6a3a5fa 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -429,20 +429,11 @@ static void free_pi_state(struct futex_pi_state *pi_state)
static struct task_struct * futex_find_get_task(pid_t pid)
struct task_struct *p;
- const struct cred *cred = current_cred(), *pcred;

p = find_task_by_vpid(pid);
- if (!p) {
- p = ERR_PTR(-ESRCH);
- } else {
- pcred = __task_cred(p);
- if (cred->euid != pcred->euid&&
- cred->euid != pcred->uid)
- p = ERR_PTR(-ESRCH);
- else
- get_task_struct(p);
- }
+ if (p)
+ get_task_struct(p);


@@ -564,8 +555,8 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
if (!pid)
return -ESRCH;
p = futex_find_get_task(pid);
- if (IS_ERR(p))
- return PTR_ERR(p);
+ if (!p)
+ return -ESRCH;

* We need to look at the task state flags to figure out,

Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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