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authorDavid Miller <davem@davemloft.net>2007-11-07 00:13:56 -0500
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-11-09 19:13:08 -0500
commit3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 (patch)
tree72f8be799a5629458aca1da877d7616d928fa00d /kernel
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 <davem@davemloft.net> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/futex_compat.c26
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,
30 return 0; 30 return 0;
31} 31}
32 32
33static void __user *futex_uaddr(struct robust_list *entry,
34 compat_long_t futex_offset)
35{
36 compat_uptr_t base = ptr_to_compat(entry);
37 void __user *uaddr = compat_ptr(base + futex_offset);
38
39 return uaddr;
40}
41
33/* 42/*
34 * Walk curr->robust_list (very carefully, it's a userspace list!) 43 * Walk curr->robust_list (very carefully, it's a userspace list!)
35 * and mark any locks found there dead, and notify any waiters. 44 * and mark any locks found there dead, and notify any waiters.
@@ -76,11 +85,12 @@ void compat_exit_robust_list(struct task_struct *curr)
76 * A pending lock might already be on the list, so 85 * A pending lock might already be on the list, so
77 * dont process it twice: 86 * dont process it twice:
78 */ 87 */
79 if (entry != pending) 88 if (entry != pending) {
80 if (handle_futex_death((void __user *)entry + futex_offset, 89 void __user *uaddr = futex_uaddr(entry, futex_offset);
81 curr, pi))
82 return;
83 90
91 if (handle_futex_death(uaddr, curr, pi))
92 return;
93 }
84 if (rc) 94 if (rc)
85 return; 95 return;
86 uentry = next_uentry; 96 uentry = next_uentry;
@@ -94,9 +104,11 @@ void compat_exit_robust_list(struct task_struct *curr)
94 104
95 cond_resched(); 105 cond_resched();
96 } 106 }
97 if (pending) 107 if (pending) {
98 handle_futex_death((void __user *)pending + futex_offset, 108 void __user *uaddr = futex_uaddr(pending, futex_offset);
99 curr, pip); 109
110 handle_futex_death(uaddr, curr, pip);
111 }
100} 112}
101 113
102asmlinkage long 114asmlinkage long