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Subject[patch 02/26] Fix compat futex hangs.
From: David Miller <>

2.6.22-stable review patch. If anyone has any objections, please let us

[FUTEX]: Fix address computation in compat code.
[ Upstream commit: 3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 ]

compat_exit_robust_list() computes a pointer to the
futex entry in userspace as follows:

(void __user *)entry + futex_offset

'entry' is a 'struct robust_list __user *', and
'futex_offset' is a 'compat_long_t' (typically a 's32').

Things explode if the 32-bit sign bit is set in futex_offset.

Type promotion sign extends futex_offset to a 64-bit value before
adding it to 'entry'.

This triggered a problem on sparc64 running 32-bit applications which
would lock up a cpu looping forever in the fault handling for the
userspace load in handle_futex_death().

Compat userspace runs with address masking (wherein the cpu zeros out
the top 32-bits of every effective address given to a memory operation
instruction) so the sparc64 fault handler accounts for this by
zero'ing out the top 32-bits of the fault address too.

Since the kernel properly uses the compat_uptr interfaces, kernel side
accesses to compat userspace work too since they will only use
addresses with the top 32-bit clear.

Because of this compat futex layer bug we get into the following loop
when executing the get_user() load near the top of handle_futex_death():

1) load from address '0xfffffffff7f16bd8', FAULT
2) fault handler clears upper 32-bits, processes fault
for address '0xf7f16bd8' which succeeds
3) goto #1
I want to thank Bernd Zeimetz, Josip Rodin, and Fabio Massimo Di Nitto
for their tireless efforts helping me track down this bug.

Signed-off-by: David S. Miller <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/futex_compat.c | 24 +++++++++++++++++++-----
1 file changed, 19 insertions(+), 5 deletions(-)
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -29,6 +29,15 @@ fetch_robust_entry(compat_uptr_t *uentry
return 0;

+static void __user *futex_uaddr(struct robust_list *entry,
+ compat_long_t futex_offset)
+ compat_uptr_t base = ptr_to_compat(entry);
+ void __user *uaddr = compat_ptr(base + futex_offset);
+ return uaddr;
* Walk curr->robust_list (very carefully, it's a userspace list!)
* and mark any locks found there dead, and notify any waiters.
@@ -61,18 +70,23 @@ void compat_exit_robust_list(struct task
if (fetch_robust_entry(&upending, &pending,
&head->list_op_pending, &pip))
- if (pending)
- handle_futex_death((void __user *)pending + futex_offset, curr, pip);
+ if (pending) {
+ void __user *uaddr = futex_uaddr(pending,
+ futex_offset);
+ handle_futex_death(uaddr, curr, pip);
+ }

while (entry != (struct robust_list __user *) &head->list) {
* A pending lock might already be on the list, so
* dont process it twice:
- if (entry != pending)
- if (handle_futex_death((void __user *)entry + futex_offset,
- curr, pi))
+ if (entry != pending) {
+ void __user *uaddr = futex_uaddr(entry,
+ futex_offset);
+ if (handle_futex_death(uaddr, curr, pi))
+ }

* Fetch the next entry in the list:
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