On Mon, Jan 07, 2019 at 10:31:34PM +0900, Masami Hiramatsu wrote: > Hello, > > On recent talk with Andrea, I started more precise investigation on > the kernel panic with kretprobes on notrace functions, which Francis > had been reported last year ( https://lkml.org/lkml/2017/7/14/466 ). > > At first, I tried to reproduce the issue. I picked up __fdget and > ftrace_ops_assist_func as probed functions. > With CONFIG_KPROBE_EVENTS_ON_NOTRACE=y, I could reproduce the kernel > panic as below. > > ===== > /sys/kernel/debug/tracing # echo "r:event_1 __fdget" >> kprobe_events > /sys/kernel/debug/tracing # echo "r:event_2 ftrace_ops_assist_func" >> > kprobe_events > /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable > [ 70.491856] BUG: unable to handle kernel NULL pointer dereference at > 0000000000000010 > [ 70.493203] PGD 800000001c62e067 P4D 800000001c62e067 PUD 1b5bf067 PMD 0 > [ 70.494247] Oops: 0010 [#1] PREEMPT SMP PTI > [ 70.494918] CPU: 6 PID: 1210 Comm: sh Not tainted 4.20.0-rc3+ #58 > [ 70.495931] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS > rel-1.10.1-0-g8891697-prebuilt.qemu-project.org 04/01/2014 > [ 70.497906] RIP: 0010:0x10 > [ 70.498465] Code: Bad RIP value. > [ 70.499077] RSP: 0018:ffffb1d4c0347e78 EFLAGS: 00010246 > [ 70.499959] RAX: 00000000fffffff7 RBX: 0000000000000000 RCX: > 0000000000000000 > [ 70.501383] RDX: ffff88f19f9c4f80 RSI: ffffffffb7d75e12 RDI: > ffffffffb7d0ede7 > [ 70.502501] RBP: 00007ffc7061af20 R08: 0000000080000002 R09: > ffff88f19f9c4f80 > [ 70.503698] R10: 0000000000000000 R11: 0000000000000000 R12: > 0000000000005401 > [ 70.504810] R13: 0000000000000000 R14: ffffb1d4c0347f58 R15: > 0000000000000000 > [ 70.506028] FS: 0000000000922880(0000) GS:ffff88f19d380000(0000) > knlGS:0000000000000000 > [ 70.507354] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 70.508271] CR2: ffffffffffffffe6 CR3: 000000001f916000 CR4: > 00000000000006e0 > [ 70.509419] DR0: 0000000000000000 DR1: 0000000000000000 DR2: > 0000000000000000 > [ 70.510803] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: > 0000000000000400 > [ 70.511748] Call Trace: > [ 70.512225] ? ksys_ioctl+0x70/0x70 > [ 70.512884] ? __fdget+0x5/0x10 > [ 70.513454] ? __fdget+0x5/0x10 > [ 70.513980] ? copy_oldmem_page_encrypted+0x20/0x20 > [ 70.514815] ? __x64_sys_ioctl+0x16/0x20 > [ 70.515596] ? do_syscall_64+0x50/0x100 > [ 70.516229] ? entry_SYSCALL_64_after_hwframe+0x49/0xbe > [ 70.517143] Modules linked in: > [ 70.517806] CR2: 0000000000000010 > [ 70.518527] ---[ end trace ece844ac05189f10 ]--- > [ 70.519417] RIP: 0010:0x10 > [ 70.520026] Code: Bad RIP value. > [ 70.520800] RSP: 0018:ffffb1d4c0347e78 EFLAGS: 00010246 > [ 70.521948] RAX: 00000000fffffff7 RBX: 0000000000000000 RCX: > 0000000000000000 > [ 70.523315] RDX: ffff88f19f9c4f80 RSI: ffffffffb7d75e12 RDI: > ffffffffb7d0ede7 > [ 70.524515] RBP: 00007ffc7061af20 R08: 0000000080000002 R09: > ffff88f19f9c4f80 > [ 70.525702] R10: 0000000000000000 R11: 0000000000000000 R12: > 0000000000005401 > [ 70.526715] R13: 0000000000000000 R14: ffffb1d4c0347f58 R15: > 0000000000000000 > [ 70.527673] FS: 0000000000922880(0000) GS:ffff88f19d380000(0000) > knlGS:0000000000000000 > [ 70.528896] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 70.529851] CR2: ffffffffffffffe6 CR3: 000000001f916000 CR4: > 00000000000006e0 > [ 70.530922] DR0: 0000000000000000 DR1: 0000000000000000 DR2: > 0000000000000000 > [ 70.531907] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: > 0000000000000400 > Killed > ===== > > This seems the kernel is trying to execute incorrect address. > > Next, I focused on the combination of probes. From Francis's report, > this issue caused by the combination of kretprobes, not kprobes. > I ensured that was true. > > r __fdget & r ftrace_ops_assist_func => NG > p __fdget & p ftrace_ops_assist_func => OK > p __fdget & r ftrace_ops_assist_func => OK > r __fdget & p ftrace_ops_assist_func => OK > > r: kretprobe, p: kprobe > > This gave me a hint of what happened. I can explain the cause of this > issue as below; > > Correct processing of kretprobe on probed-function. > > <caller> > -><probed-function> > ->fentry > ->ftrace_ops_assist_func() > ->kprobe_ftrace_handler() > ...->pre_handler_kretprobe() > push the return address (caller) of probed-function to top of the > kretprobe list and replace it with kretprobe_trampoline. > <-(ftrace_ops_assist_func()) > <-(fentry) > <-(probed-function) > [kretprobe_trampoline] > ->tampoline_handler() > pop the return address (caller) from top of the kretprobe list > <-(trampoline_handler()) > <caller> > > When we put a kretprobe on ftrace_ops_assist_func(), below happens > > <caller> > -><probed-function> > ->fentry > ->ftrace_ops_assist_func() > ->int3 > ->kprobe_int3_handler() > ...->pre_handler_kretprobe() > push the return address (*fentry*) of ftrace_ops_assist_func() to > top of the kretprobe list and replace it with kretprobe_trampoline. > <-kprobe_int3_handler() > <-(int3) > ->kprobe_ftrace_handler() > ...->pre_handler_kretprobe() > push the return address (caller) of probed-function to top of the > kretprobe list and replace it with kretprobe_trampoline. > <-(kprobe_ftrace_handler()) > <-(ftrace_ops_assist_func()) > [kretprobe_trampoline] > ->tampoline_handler() > pop the return address (caller) from top of the kretprobe list > <-(trampoline_handler()) > <caller> > [run caller with incorrect stack information] > <-(<caller>) > !!KERNEL PANIC!! > > Therefore, this kernel panic happens only when we put 2 k*ret*probes on > ftrace_ops_assist_func() and other functions. If we put kprobes, it > doesn't cause any issue, since it doesn't change the return address. > > To fix (or just avoid) this issue, we can introduce a frame pointer > verification to skip wrong order entries. And I also would like to > blacklist those functions because those are part of ftrace-based > kprobe handling routine. > > BTW, this is not all of issues. To remove CONFIG_KPROBE_EVENTS_ON_NOTRACE > I'm trying to find out other notrace functions which can cause > kernel crash by probing. Mostly done on x86, so I'll post it > after this series. > > Thank you,
Apart than the missing include <linux/kprobes.h> in PATCH 2/2 everything else looks good to me. Tested-by: Andrea Righi <[email protected]> Thanks! -Andrea
