Messages in this thread Patch in this message | | | Date | Tue, 06 Nov 2007 21:13:56 -0800 (PST) | Subject | Re: Fix for sparc64 cpu hangs. | From | David Miller <> |
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From: David Miller <davem@davemloft.net> Date: Tue, 06 Nov 2007 20:34:33 -0800 (PST)
> [FUTEX]: Fix address computation in compat code.
Sorry, I just noticed there is a second handle_futex_death() call in compat_exit_robust_list() which has the same address computation bug.
Here is an updated patch:
[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>
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c index 00b5726..1931457 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 +85,13 @@ 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) return; uentry = next_uentry; @@ -94,9 +105,11 @@ void compat_exit_robust_list(struct task_struct *curr) 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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