Commit 3c5fd9c7 authored Nov 06, 2007 by David Miller Committed by Linus Torvalds Nov 09, 2007

[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>

parent 8fc543c8

kernel/futex_compat.c

+19 −7

Original line number	Diff line number	Diff line
		@@ -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