[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 <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 00b5726..0a43def 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -30,6 +30,15 @@
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 @@
* 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)
return;
uentry = next_uentry;
@@ -94,9 +104,11 @@
cond_resched();
}
- 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
