Messages in this thread | | | Date | Mon, 28 Jan 2019 06:15:53 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [BUG] racy access to p->mm in membarrier_global_expedited() |
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On Fri, Jan 25, 2019 at 06:26:47PM +0100, Jann Horn wrote: > membarrier_global_expedited() runs the following code (introduced in > commit c5f58bd58f43), protected only by an RCU read-side critical > section and the cpu_hotplug_lock: > > p = task_rcu_dereference(&cpu_rq(cpu)->curr); > if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { > if (!fallback) > __cpumask_set_cpu(cpu, tmpmask); > else > smp_call_function_single(cpu, ipi_mb, NULL, 1); > } > > p->mm is not protected by either lock. This means that in theory, the > following races could occur: > > 1. If the compiler emitted two separate reads of ->mm, the second read > of p->mm could return a NULL pointer and crash. > 2. If the mm is deallocated directly before the atomic_read() occurs, > the atomic_read() could access a freed pointer (I think?). > > Neither of these are particularly likely - looking at the assembly of > a normal build, the first race doesn't exist because the compiler > optimizes the second read away, and the second race isn't going to > cause anything particularly interesting. Still, this should probably > be fixed... > > As far as I can tell, you'll have to either take the task_lock() > around the "p->mm && (atomic_read(&p->mm->membarrier_state)" or add > RCU to the lifetime of mm_struct. I'm not entirely sure what the > better fix is... probably task_lock() makes more sense?
Ouch!!!
Acquiring task_lock() would work, but would be a global lock. This could be addressed to some extent by batching concurrent membarrier_global_expedited() invocations, so that one call to membarrier_global_expedited() does the job for the set of concurrent calls. The usual approach would use a counter, a pair of wait queues, and a kthread.
I must defer to the mm guys on adding RCU to the lifetime of mm_struct.
Another approach would be to put the MEMBARRIER_STATE_GLOBAL_EXPEDITED in the task structure. Yet another approach would be to acquire the runqueue lock, thus preventing the task from switching away -- except that it might be in the middle of exit(), so never mind.
Other approaches?
Thanx, Paul
> To test the bug, I patched an extra delay into the code: > > ==================== > diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c > index 3cd8a3a795d2..69cc52039576 100644 > --- a/kernel/sched/membarrier.c > +++ b/kernel/sched/membarrier.c > @@ -14,6 +14,7 @@ > * GNU General Public License for more details. > */ > #include "sched.h" > +#include <linux/delay.h> > > /* > * Bitmask made from a "or" of all commands within enum membarrier_cmd, > @@ -81,7 +82,7 @@ static int membarrier_global_expedited(void) > > rcu_read_lock(); > p = task_rcu_dereference(&cpu_rq(cpu)->curr); > - if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & > + if (p && p->mm && (mdelay(100), 1) && > (atomic_read(&p->mm->membarrier_state) & > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { > if (!fallback) > __cpumask_set_cpu(cpu, tmpmask); > ==================== > > On a kernel with that patch applied, I ran this test code: > > ==================== > #define _GNU_SOURCE > #include <unistd.h> > #include <sys/syscall.h> > #include <stdio.h> > #include <linux/membarrier.h> > #include <err.h> > > int main(void) { > while (1) { > printf("executing global expedited barrier...\n"); > int res = syscall(__NR_membarrier, MEMBARRIER_CMD_GLOBAL_EXPEDITED, 0); > if (res) err(1, "barrier"); > } > } > ==================== > > That resulted in this splat: > > [ 212.697681] ================================================================== > [ 212.700582] BUG: KASAN: null-ptr-deref in > membarrier_global_expedited+0x15f/0x220 > [ 212.703346] Read of size 4 at addr 0000000000000378 by task barrier/1177 > > [ 212.706384] CPU: 1 PID: 1177 Comm: barrier Not tainted 5.0.0-rc3+ #246 > [ 212.708925] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), > BIOS 1.10.2-1 04/01/2014 > [ 212.712263] Call Trace: > [ 212.713177] dump_stack+0x71/0xab > [ 212.714375] ? membarrier_global_expedited+0x15f/0x220 > [ 212.716236] ? membarrier_global_expedited+0x15f/0x220 > [ 212.718099] kasan_report+0x176/0x192 > [ 212.719445] ? finish_task_switch+0x340/0x3d0 > [ 212.721057] ? membarrier_global_expedited+0x15f/0x220 > [ 212.722921] membarrier_global_expedited+0x15f/0x220 > [ 212.724696] ? ipi_mb+0x10/0x10 > [ 212.725816] ? vfs_write+0x120/0x230 > [ 212.727113] ? __ia32_sys_read+0x50/0x50 > [ 212.728596] __x64_sys_membarrier+0x85/0xf0 > [ 212.730056] do_syscall_64+0x73/0x160 > [ 212.731428] entry_SYSCALL_64_after_hwframe+0x44/0xa9 > [ 212.733236] RIP: 0033:0x7fbe8747e229 > [ 212.734540] Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 > 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 > 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 3f 4c 2b 00 f7 d8 64 89 > 01 48 > [ 212.741109] RSP: 002b:00007fffcb62a7c8 EFLAGS: 00000202 ORIG_RAX: > 0000000000000144 > [ 212.743831] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fbe8747e229 > [ 212.746335] RDX: 00007fbe87475730 RSI: 0000000000000000 RDI: 0000000000000002 > [ 212.748855] RBP: 00007fffcb62a7e0 R08: 00007fffcb62a8c0 R09: 00007fffcb62a8c0 > [ 212.751374] R10: 00007fbe8793c700 R11: 0000000000000202 R12: 0000563ee2ac9610 > [ 212.753842] R13: 00007fffcb62a8c0 R14: 0000000000000000 R15: 0000000000000000 > [ 212.756305] ================================================================== >
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