From 4d5898f8b021d02a05059fd3e78500f94c66dc50 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Tue, 30 Apr 2019 13:34:51 +0100 Subject: [PATCH] io_uring: fix SQPOLL cpu validation In io_sq_offload_start(), we call cpu_possible() on an unbounded cpu value from userspace. On v5.1-rc7 on arm64 with CONFIG_DEBUG_PER_CPU_MAPS, this results in a splat: WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 cpu_max_bits_warn include/linux/cpumask.h:121 [inline] There was an attempt to fix this in commit: bbd1dc597beb5c27 ("io_uring: only test SQPOLL cpu after we've verified it") ... by adding a check after the cpu value had been limited to NR_CPU_IDS using array_index_nospec(). However, this left an unbound check at the start of the function, for which the warning still fires. Let's fix this correctly by checking that the cpu value is bound by nr_cpu_ids before passing it to cpu_possible(). Note that only nr_cpu_ids of a cpumask are guaranteed to exist at runtime, and nr_cpu_ids can be significantly smaller than NR_CPUs. For example, an arm64 defconfig has NR_CPUS=256, while my test VM has 4 vCPUs. Following the intent from the commit message for bbd1dc597beb5c27, the check is moved under the SQ_AFF branch, which is the only branch where the cpu values is consumed. The check is performed before bounding the value with array_index_nospec() so that we don't silently accept bogus cpu values from userspace, where array_index_nospec() would force these values to 0. I suspect we can remove the array_index_nospec() call entirely, but I've conservatively left that in place, updated to use nr_cpu_ids to match the prior check. Tested on arm64 with the Syzkaller reproducer: https://syzkaller.appspot.com/bug?extid=cd714a07c6de2bc34293 https://syzkaller.appspot.com/x/repro.syz?x=15d8b397200000 Full splat from before this patch: WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 cpu_max_bits_warn include/linux/cpumask.h:121 [inline] WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 cpumask_check include/linux/cpumask.h:128 [inline] WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 cpumask_test_cpu include/linux/cpumask.h:344 [inline] WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 io_sq_offload_start fs/io_uring.c:2244 [inline] WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 io_uring_create fs/io_uring.c:2864 [inline] WARNING: CPU: 1 PID: 27601 at include/linux/cpumask.h:121 io_uring_setup+0x1108/0x15a0 fs/io_uring.c:2916 Kernel panic - not syncing: panic_on_warn set ... CPU: 1 PID: 27601 Comm: syz-executor.0 Not tainted 5.1.0-rc7 #3 Hardware name: linux,dummy-virt (DT) Call trace: dump_backtrace+0x0/0x2f0 include/linux/compiler.h:193 show_stack+0x20/0x30 arch/arm64/kernel/traps.c:158 __dump_stack lib/dump_stack.c:77 [inline] dump_stack+0x110/0x190 lib/dump_stack.c:113 panic+0x384/0x68c kernel/panic.c:214 __warn+0x2bc/0x2c0 kernel/panic.c:571 report_bug+0x228/0x2d8 lib/bug.c:186 bug_handler+0xa0/0x1a0 arch/arm64/kernel/traps.c:956 call_break_hook arch/arm64/kernel/debug-monitors.c:301 [inline] brk_handler+0x1d4/0x388 arch/arm64/kernel/debug-monitors.c:316 do_debug_exception+0x1a0/0x468 arch/arm64/mm/fault.c:831 el1_dbg+0x18/0x8c cpu_max_bits_warn include/linux/cpumask.h:121 [inline] cpumask_check include/linux/cpumask.h:128 [inline] cpumask_test_cpu include/linux/cpumask.h:344 [inline] io_sq_offload_start fs/io_uring.c:2244 [inline] io_uring_create fs/io_uring.c:2864 [inline] io_uring_setup+0x1108/0x15a0 fs/io_uring.c:2916 __do_sys_io_uring_setup fs/io_uring.c:2929 [inline] __se_sys_io_uring_setup fs/io_uring.c:2926 [inline] __arm64_sys_io_uring_setup+0x50/0x70 fs/io_uring.c:2926 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline] invoke_syscall arch/arm64/kernel/syscall.c:47 [inline] el0_svc_common.constprop.0+0x148/0x2e0 arch/arm64/kernel/syscall.c:83 el0_svc_handler+0xdc/0x100 arch/arm64/kernel/syscall.c:129 el0_svc+0x8/0xc arch/arm64/kernel/entry.S:948 SMP: stopping secondary CPUs Dumping ftrace buffer: (ftrace buffer empty) Kernel Offset: disabled CPU features: 0x002,23000438 Memory Limit: none Rebooting in 1 seconds.. Fixes: bbd1dc597beb5c27 ("io_uring: only test SQPOLL cpu after we've verified it") Signed-off-by: Mark Rutland Cc: Jens Axboe Cc: Alexander Viro Cc: linux-block@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Simplied the logic Signed-off-by: Jens Axboe --- fs/io_uring.c | 10 +++------- 1 file changed, 3 insertions(+), 7 deletions(-) diff --git a/fs/io_uring.c b/fs/io_uring.c index 0a894d7baceb0..5954047ee96d9 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -2319,10 +2319,6 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx, mmgrab(current->mm); ctx->sqo_mm = current->mm; - ret = -EINVAL; - if (!cpu_possible(p->sq_thread_cpu)) - goto err; - if (ctx->flags & IORING_SETUP_SQPOLL) { ret = -EPERM; if (!capable(CAP_SYS_ADMIN)) @@ -2333,11 +2329,11 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx, ctx->sq_thread_idle = HZ; if (p->flags & IORING_SETUP_SQ_AFF) { - int cpu; + int cpu = array_index_nospec(p->sq_thread_cpu, + nr_cpu_ids); - cpu = array_index_nospec(p->sq_thread_cpu, NR_CPUS); ret = -EINVAL; - if (!cpu_possible(p->sq_thread_cpu)) + if (!cpu_possible(cpu)) goto err; ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread, -- 2.39.5