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authorSteven Rostedt <srostedt@redhat.com>2007-12-05 09:46:09 -0500
committerIngo Molnar <mingo@elte.hu>2007-12-05 09:46:09 -0500
commitce6bd420f43b28038a2c6e8fbb86ad24014727b6 (patch)
treee35e86507e11c375c2668a0391f4cf66bb581783 /kernel/futex.c
parente3c0ac04f980750a368f7cd5f1b8d1d2cdc1f735 (diff)
futex: fix for futex_wait signal stack corruption
David Holmes found a bug in the -rt tree with respect to pthread_cond_timedwait. After trying his test program on the latest git from mainline, I found the bug was there too. The bug he was seeing that his test program showed, was that if one were to do a "Ctrl-Z" on a process that was in the pthread_cond_timedwait, and then did a "bg" on that process, it would return with a "-ETIMEDOUT" but early. That is, the timer would go off early. Looking into this, I found the source of the problem. And it is a rather nasty bug at that. Here's the relevant code from kernel/futex.c: (not in order in the file) [...] smlinkage long sys_futex(u32 __user *uaddr, int op, u32 val, struct timespec __user *utime, u32 __user *uaddr2, u32 val3) { struct timespec ts; ktime_t t, *tp = NULL; u32 val2 = 0; int cmd = op & FUTEX_CMD_MASK; if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI)) { if (copy_from_user(&ts, utime, sizeof(ts)) != 0) return -EFAULT; if (!timespec_valid(&ts)) return -EINVAL; t = timespec_to_ktime(ts); if (cmd == FUTEX_WAIT) t = ktime_add(ktime_get(), t); tp = &t; } [...] return do_futex(uaddr, op, val, tp, uaddr2, val2, val3); } [...] long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, u32 __user *uaddr2, u32 val2, u32 val3) { int ret; int cmd = op & FUTEX_CMD_MASK; struct rw_semaphore *fshared = NULL; if (!(op & FUTEX_PRIVATE_FLAG)) fshared = &current->mm->mmap_sem; switch (cmd) { case FUTEX_WAIT: ret = futex_wait(uaddr, fshared, val, timeout); [...] static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared, u32 val, ktime_t *abs_time) { [...] struct restart_block *restart; restart = &current_thread_info()->restart_block; restart->fn = futex_wait_restart; restart->arg0 = (unsigned long)uaddr; restart->arg1 = (unsigned long)val; restart->arg2 = (unsigned long)abs_time; restart->arg3 = 0; if (fshared) restart->arg3 |= ARG3_SHARED; return -ERESTART_RESTARTBLOCK; [...] static long futex_wait_restart(struct restart_block *restart) { u32 __user *uaddr = (u32 __user *)restart->arg0; u32 val = (u32)restart->arg1; ktime_t *abs_time = (ktime_t *)restart->arg2; struct rw_semaphore *fshared = NULL; restart->fn = do_no_restart_syscall; if (restart->arg3 & ARG3_SHARED) fshared = &current->mm->mmap_sem; return (long)futex_wait(uaddr, fshared, val, abs_time); } So when the futex_wait is interrupt by a signal we break out of the hrtimer code and set up or return from signal. This code does not return back to userspace, so we set up a RESTARTBLOCK. The bug here is that we save the "abs_time" which is a pointer to the stack variable "ktime_t t" from sys_futex. This returns and unwinds the stack before we get to call our signal. On return from the signal we go to futex_wait_restart, where we update all the parameters for futex_wait and call it. But here we have a problem where abs_time is no longer valid. I verified this with print statements, and sure enough, what abs_time was set to ends up being garbage when we get to futex_wait_restart. The solution I did to solve this (with input from Linus Torvalds) was to add unions to the restart_block to allow system calls to use the restart with specific parameters. This way the futex code now saves the time in a 64bit value in the restart block instead of storing it on the stack. Note: I'm a bit nervious to add "linux/types.h" and use u32 and u64 in thread_info.h, when there's a #ifdef __KERNEL__ just below that. Not sure what that is there for. If this turns out to be a problem, I've tested this with using "unsigned int" for u32 and "unsigned long long" for u64 and it worked just the same. I'm using u32 and u64 just to be consistent with what the futex code uses. Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/futex.c')
-rw-r--r--kernel/futex.c25
1 files changed, 13 insertions, 12 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 9dc591ab681a..e8fbdd7d95ac 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1149,9 +1149,9 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
1149 1149
1150/* 1150/*
1151 * In case we must use restart_block to restart a futex_wait, 1151 * In case we must use restart_block to restart a futex_wait,
1152 * we encode in the 'arg3' shared capability 1152 * we encode in the 'flags' shared capability
1153 */ 1153 */
1154#define ARG3_SHARED 1 1154#define FLAGS_SHARED 1
1155 1155
1156static long futex_wait_restart(struct restart_block *restart); 1156static long futex_wait_restart(struct restart_block *restart);
1157 1157
@@ -1290,12 +1290,13 @@ static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
1290 struct restart_block *restart; 1290 struct restart_block *restart;
1291 restart = &current_thread_info()->restart_block; 1291 restart = &current_thread_info()->restart_block;
1292 restart->fn = futex_wait_restart; 1292 restart->fn = futex_wait_restart;
1293 restart->arg0 = (unsigned long)uaddr; 1293 restart->futex.uaddr = (u32 *)uaddr;
1294 restart->arg1 = (unsigned long)val; 1294 restart->futex.val = val;
1295 restart->arg2 = (unsigned long)abs_time; 1295 restart->futex.time = abs_time->tv64;
1296 restart->arg3 = 0; 1296 restart->futex.flags = 0;
1297
1297 if (fshared) 1298 if (fshared)
1298 restart->arg3 |= ARG3_SHARED; 1299 restart->futex.flags |= FLAGS_SHARED;
1299 return -ERESTART_RESTARTBLOCK; 1300 return -ERESTART_RESTARTBLOCK;
1300 } 1301 }
1301 1302
@@ -1310,15 +1311,15 @@ static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
1310 1311
1311static long futex_wait_restart(struct restart_block *restart) 1312static long futex_wait_restart(struct restart_block *restart)
1312{ 1313{
1313 u32 __user *uaddr = (u32 __user *)restart->arg0; 1314 u32 __user *uaddr = (u32 __user *)restart->futex.uaddr;
1314 u32 val = (u32)restart->arg1;
1315 ktime_t *abs_time = (ktime_t *)restart->arg2;
1316 struct rw_semaphore *fshared = NULL; 1315 struct rw_semaphore *fshared = NULL;
1316 ktime_t t;
1317 1317
1318 t.tv64 = restart->futex.time;
1318 restart->fn = do_no_restart_syscall; 1319 restart->fn = do_no_restart_syscall;
1319 if (restart->arg3 & ARG3_SHARED) 1320 if (restart->futex.flags & FLAGS_SHARED)
1320 fshared = &current->mm->mmap_sem; 1321 fshared = &current->mm->mmap_sem;
1321 return (long)futex_wait(uaddr, fshared, val, abs_time); 1322 return (long)futex_wait(uaddr, fshared, restart->futex.val, &t);
1322} 1323}
1323 1324
1324 1325