path: root/kernel/futex_compat.c
diff options
authorDavid Miller <>2007-11-06 21:13:56 -0800
committerLinus Torvalds <>2007-11-09 16:13:08 -0800
commit3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 (patch)
tree72f8be799a5629458aca1da877d7616d928fa00d /kernel/futex_compat.c
parent8fc543c8f004fc9dfe0a262dc452dfe2eca4589b (diff)
[FUTEX] Fix address computation in compat code.
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 <>
Diffstat (limited to 'kernel/futex_compat.c')
1 files changed, 19 insertions, 7 deletions
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 00b572666cc7..0a43def6fee7 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -30,6 +30,15 @@ fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
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.
@@ -76,11 +85,12 @@ void compat_exit_robust_list(struct task_struct *curr)
* 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))
- return;
+ if (entry != pending) {
+ void __user *uaddr = futex_uaddr(entry, futex_offset);
+ if (handle_futex_death(uaddr, curr, pi))
+ return;
+ }
if (rc)
uentry = next_uentry;
@@ -94,9 +104,11 @@ void compat_exit_robust_list(struct task_struct *curr)
- 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);
+ }
asmlinkage long