Re: [PATCH, take4] FUTEX : new PRIVATE futexes

Eric Dumazet
2007-04-10T09:49:18Z

On Sat, 7 Apr 2007 15:15:56 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Sat, 7 Apr 2007 10:43:39 +0200 Eric Dumazet <dada1@cosmosbay.com> wrote:
> >
> > get_futex_key() does a check against sizeof(u32) regardless of futex being 64bits or not.
> > So it is possible a 64bit futex spans two pages of memory...
> > I had to change get_futex_key() prototype to be able to do a correct test.
> >
>
> Cold we please have that in a separate patch? It's logically a part of the
> 64-bit-futex work, is it not?

Yes you probably want this patch to fix 64bit futexes support.

[PATCH] get_futex_key() must check proper alignement for 64bit futexes

get_futex_key() does an alignment check against sizeof(u32) regardless of futex
being 64bits or not.

So it is possible a 64bit futex spans two pages of memory, and some
malicious user code can trigger data corruption.

We must add a 'fsize' parameter to get_futex_key(), telling it the size of the
futex (4 or 8 bytes)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
include/linux/futex.h | 2 -
kernel/futex.c | 78 ++++++++++++++++++++++++++--------------
2 files changed, 52 insertions(+), 28 deletions(-)

--- linux-2.6.21-rc6-mm1/kernel/futex.c
+++ linux-2.6.21-rc6-mm1-ed/kernel/futex.c
@@ -189,19 +189,22 @@ static inline int match_futex(union fute
&& key1->both.offset == key2->both.offset);
}

-/*
- * Get parameters which are the keys for a futex.
+/**
+ * get_futex_key - Get parameters which are the keys for a futex.
+ * @uaddr: virtual address of the futex
+ * @size: size of futex (4 or 8)
+ * @key: address where result is stored.
+ *
+ * Returns a negative error code or 0
+ * The key words are stored in *key on success.
*
* For shared mappings, it's (page->index, vma->vm_file->f_path.dentry->d_inode,
* offset_within_page). For private mappings, it's (uaddr, current->mm).
* We can usually work out the index without swapping in the page.
*
- * Returns: 0, or negative error code.
- * The key words are stored in *key on success.
- *
* Should be called with &current->mm->mmap_sem but NOT any spinlocks.
*/
-int get_futex_key(void __user *uaddr, union futex_key *key)
+int get_futex_key(void __user *uaddr, int size, union futex_key *key)
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
@@ -213,7 +216,7 @@ int get_futex_key(void __user *uaddr, un
* The futex address must be "naturally" aligned.
*/
key->both.offset = address % PAGE_SIZE;
- if (unlikely((key->both.offset % sizeof(u32)) != 0))
+ if (unlikely((key->both.offset & (size - 1)) != 0))
return -EINVAL;
address -= key->both.offset;

@@ -705,17 +708,18 @@ double_lock_hb(struct futex_hash_bucket
* Wake up all waiters hashed on the physical page that is mapped
* to this virtual address:
*/
-static int futex_wake(unsigned long __user *uaddr, int nr_wake)
+static int futex_wake(unsigned long __user *uaddr, int futex64, int nr_wake)
{
struct futex_hash_bucket *hb;
struct futex_q *this, *next;
struct plist_head *head;
union futex_key key;
int ret;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

down_read(&current->mm->mmap_sem);

- ret = get_futex_key(uaddr, &key);
+ ret = get_futex_key(uaddr, fsize, &key);
if (unlikely(ret != 0))
goto out;

@@ -817,6 +821,7 @@ futex_requeue_pi(unsigned long __user *u
struct rt_mutex_waiter *waiter, *top_waiter = NULL;
struct rt_mutex *lock2 = NULL;
int ret, drop_count = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

if (refill_pi_state_cache())
return -ENOMEM;
@@ -827,10 +832,10 @@ retry:
*/
down_read(&current->mm->mmap_sem);

- ret = get_futex_key(uaddr1, &key1);
+ ret = get_futex_key(uaddr1, fsize, &key1);
if (unlikely(ret != 0))
goto out;
- ret = get_futex_key(uaddr2, &key2);
+ ret = get_futex_key(uaddr2, fsize, &key2);
if (unlikely(ret != 0))
goto out;

@@ -1005,14 +1010,15 @@ futex_wake_op(unsigned long __user *uadd
struct plist_head *head;
struct futex_q *this, *next;
int ret, op_ret, attempt = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

retryfull:
down_read(&current->mm->mmap_sem);

- ret = get_futex_key(uaddr1, &key1);
+ ret = get_futex_key(uaddr1, fsize, &key1);
if (unlikely(ret != 0))
goto out;
- ret = get_futex_key(uaddr2, &key2);
+ ret = get_futex_key(uaddr2, fsize, &key2);
if (unlikely(ret != 0))
goto out;

@@ -1125,14 +1131,15 @@ futex_requeue(unsigned long __user *uadd
struct plist_head *head1;
struct futex_q *this, *next;
int ret, drop_count = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

retry:
down_read(&current->mm->mmap_sem);

- ret = get_futex_key(uaddr1, &key1);
+ ret = get_futex_key(uaddr1, fsize, &key1);
if (unlikely(ret != 0))
goto out;
- ret = get_futex_key(uaddr2, &key2);
+ ret = get_futex_key(uaddr2, fsize, &key2);
if (unlikely(ret != 0))
goto out;

@@ -1390,9 +1397,15 @@ static int fixup_pi_state_owner(unsigned
return ret;
}

+/*
+ * In case we must use restart_block to restart a futex_wait,
+ * we encode in the 'arg3' futex64 capability
+ */
+#define ARG3_FUTEX64 1
+
static long futex_wait_restart(struct restart_block *restart);
-static int futex_wait(unsigned long __user *uaddr, unsigned long val,
- ktime_t *abs_time, int futex64)
+static int futex_wait(unsigned long __user *uaddr, int futex64,
+ unsigned long val, ktime_t *abs_time)
{
struct task_struct *curr = current;
DECLARE_WAITQUEUE(wait, curr);
@@ -1402,12 +1415,13 @@ static int futex_wait(unsigned long __us
int ret;
struct hrtimer_sleeper t, *to = NULL;
int rem = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

q.pi_state = NULL;
retry:
down_read(&curr->mm->mmap_sem);

- ret = get_futex_key(uaddr, &q.key);
+ ret = get_futex_key(uaddr, fsize, &q.key);
if (unlikely(ret != 0))
goto out_release_sem;

@@ -1597,7 +1611,11 @@ static int futex_wait(unsigned long __us
restart->arg0 = (unsigned long)uaddr;
restart->arg1 = val;
restart->arg2 = (unsigned long)abs_time;
- restart->arg3 = (unsigned long)futex64;
+ restart->arg3 = 0;
+#ifdef CONFIG_64BIT
+ if (futex64)
+ restart->arg3 |= ARG3_FUTEX64;
+#endif
return -ERESTART_RESTARTBLOCK;
}

@@ -1615,10 +1633,14 @@ static long futex_wait_restart(struct re
unsigned long __user *uaddr = (unsigned long __user *)restart->arg0;
unsigned long val = restart->arg1;
ktime_t *abs_time = (ktime_t *)restart->arg2;
- int futex64 = (int)restart->arg3;
+ int futex64 = 0;

+#ifdef CONFIG_64BIT
+ if (restart->arg3 & ARG3_FUTEX64)
+ futex64 = 1;
+#endif
restart->fn = do_no_restart_syscall;
- return (long)futex_wait(uaddr, val, abs_time, futex64);
+ return (long)futex_wait(uaddr, futex64, val, abs_time);
}

@@ -1682,6 +1704,7 @@ static int futex_lock_pi(unsigned long _
unsigned long uval, newval, curval;
struct futex_q q;
int ret, lock_held, attempt = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

if (refill_pi_state_cache())
return -ENOMEM;
@@ -1697,7 +1720,7 @@ static int futex_lock_pi(unsigned long _
retry:
down_read(&curr->mm->mmap_sem);

- ret = get_futex_key(uaddr, &q.key);
+ ret = get_futex_key(uaddr, fsize, &q.key);
if (unlikely(ret != 0))
goto out_release_sem;

@@ -1907,6 +1930,7 @@ static int futex_unlock_pi(unsigned long
struct plist_head *head;
union futex_key key;
int ret, attempt = 0;
+ int fsize = futex64 ? sizeof(u64) : sizeof(u32);

retry:
if (futex_get_user(&uval, uaddr, futex64))
@@ -1921,7 +1945,7 @@ retry:
*/
down_read(&current->mm->mmap_sem);

- ret = get_futex_key(uaddr, &key);
+ ret = get_futex_key(uaddr, fsize, &key);
if (unlikely(ret != 0))
goto out;

@@ -2094,7 +2118,7 @@ static int futex_fd(u32 __user *uaddr, i
q->pi_state = NULL;

down_read(&current->mm->mmap_sem);
- err = get_futex_key(uaddr, &q->key);
+ err = get_futex_key(uaddr, sizeof(u32), &q->key);

if (unlikely(err != 0)) {
up_read(&current->mm->mmap_sem);
@@ -2238,7 +2262,7 @@ retry:
*/
if (!pi) {
if (uval & FUTEX_WAITERS)
- futex_wake((unsigned long __user *)uaddr, 1);
+ futex_wake((unsigned long __user *)uaddr, 0, 1);
}
}
return 0;
@@ -2329,10 +2353,10 @@ long do_futex(unsigned long __user *uadd

switch (op) {
case FUTEX_WAIT:
- ret = futex_wait(uaddr, val, timeout, fut64);
+ ret = futex_wait(uaddr, fut64, val, timeout);
break;
case FUTEX_WAKE:
- ret = futex_wake(uaddr, val);
+ ret = futex_wake(uaddr, fut64, val);
break;
case FUTEX_FD:
if (fut64)
--- linux-2.6.21-rc6-mm1/include/linux/futex.h
+++ linux-2.6.21-rc6-mm1-ed/include/linux/futex.h
@@ -135,7 +135,7 @@ union futex_key {
int offset;
} both;
};
-int get_futex_key(void __user *uaddr, union futex_key *key);
+int get_futex_key(void __user *uaddr, int size, union futex_key *key);
void get_futex_key_refs(union futex_key *key);
void drop_futex_key_refs(union futex_key *key);