From 3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 Mon Sep 17 00:00:00 2001 From: David Miller Date: Tue, 6 Nov 2007 21:13:56 -0800 Subject: [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 Signed-off-by: Linus Torvalds --- kernel/futex_compat.c | 26 +++++++++++++++++++------- 1 file 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) return; uentry = next_uentry; @@ -94,9 +104,11 @@ 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 -- 2.20.1