Re: Oops caused by race between livepatch and ftrace

From: Josh Poimboeuf
Date: Wed May 29 2019 - 13:42:52 EST

On Wed, May 29, 2019 at 07:29:04PM +0200, Jessica Yu wrote:
> +++ Josh Poimboeuf [21/05/19 11:42 -0500]:
> > On Tue, May 21, 2019 at 10:42:04AM -0400, Steven Rostedt wrote:
> > > On Tue, 21 May 2019 09:16:29 -0500
> > > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > >
> > > > > Hmm, this may blow up with lockdep, as I believe we already have a
> > > > > locking dependency of:
> > > > >
> > > > > text_mutex -> ftrace_lock
> > > > >
> > > > > And this will reverses it. (kprobes appears to take the locks in this
> > > > > order).
> > > > >
> > > > > Perhaps have live kernel patching grab ftrace_lock?
> > > >
> > > > Where does kprobes call into ftrace with the text_mutex? I couldn't
> > > > find it.
> > >
> > > Hmm, maybe it doesn't. I was looking at the arm_kprobe_ftrace() but
> > > it doesn't call it with text_mutex().
> > >
> > > Maybe it is fine, but we had better perform a lot of testing with
> > > lockdep on to make sure.
> >
> > Hm. I suppose using ftrace_lock might be less risky since that lock is
> > only used internally by ftrace (up until now). But I think it would
> > also make less sense because the text_mutex is supposed to protect code
> > patching. And presumably ftrace_lock is supposed to be ftrace-specific.
> >
> > Here's the latest patch, still using text_mutex. I added some lockdep
> > assertions to ensure the permissions toggling functions are always
> > called with text_mutex. It's running through 0-day right now. I can
> > try to run it through various tests with CONFIG_LOCKDEP.
> >
> >
> > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Subject: [PATCH] livepatch: Fix ftrace module text permissions race
> >
> > It's possible for livepatch and ftrace to be toggling a module's text
> > permissions at the same time, resulting in the following panic:
> >
> > BUG: unable to handle page fault for address: ffffffffc005b1d9
> > #PF: supervisor write access in kernel mode
> > #PF: error_code(0x0003) - permissions violation
> > PGD 3ea0c067 P4D 3ea0c067 PUD 3ea0e067 PMD 3cc13067 PTE 3b8a1061
> > Oops: 0003 [#1] PREEMPT SMP PTI
> > CPU: 1 PID: 453 Comm: insmod Tainted: G O K 5.2.0-rc1-a188339ca5 #1
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
> > RIP: 0010:apply_relocate_add+0xbe/0x14c
> > Code: fa 0b 74 21 48 83 fa 18 74 38 48 83 fa 0a 75 40 eb 08 48 83 38 00 74 33 eb 53 83 38 00 75 4e 89 08 89 c8 eb 0a 83 38 00 75 43 <89> 08 48 63 c1 48 39 c8 74 2e eb 48 83 38 00 75 32 48 29 c1 89 08
> > RSP: 0018:ffffb223c00dbb10 EFLAGS: 00010246
> > RAX: ffffffffc005b1d9 RBX: 0000000000000000 RCX: ffffffff8b200060
> > RDX: 000000000000000b RSI: 0000004b0000000b RDI: ffff96bdfcd33000
> > RBP: ffffb223c00dbb38 R08: ffffffffc005d040 R09: ffffffffc005c1f0
> > R10: ffff96bdfcd33c40 R11: ffff96bdfcd33b80 R12: 0000000000000018
> > R13: ffffffffc005c1f0 R14: ffffffffc005e708 R15: ffffffff8b2fbc74
> > FS: 00007f5f447beba8(0000) GS:ffff96bdff900000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: ffffffffc005b1d9 CR3: 000000003cedc002 CR4: 0000000000360ea0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> > klp_init_object_loaded+0x10f/0x219
> > ? preempt_latency_start+0x21/0x57
> > klp_enable_patch+0x662/0x809
> > ? virt_to_head_page+0x3a/0x3c
> > ? kfree+0x8c/0x126
> > patch_init+0x2ed/0x1000 [livepatch_test02]
> > ? 0xffffffffc0060000
> > do_one_initcall+0x9f/0x1c5
> > ? kmem_cache_alloc_trace+0xc4/0xd4
> > ? do_init_module+0x27/0x210
> > do_init_module+0x5f/0x210
> > load_module+0x1c41/0x2290
> > ? fsnotify_path+0x3b/0x42
> > ? strstarts+0x2b/0x2b
> > ? kernel_read+0x58/0x65
> > __do_sys_finit_module+0x9f/0xc3
> > ? __do_sys_finit_module+0x9f/0xc3
> > __x64_sys_finit_module+0x1a/0x1c
> > do_syscall_64+0x52/0x61
> > entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > The above panic occurs when loading two modules at the same time with
> > ftrace enabled, where at least one of the modules is a livepatch module:
> >
> > CPU0 CPU1
> > klp_enable_patch()
> > klp_init_object_loaded()
> > module_disable_ro()
> > ftrace_module_enable()
> > ftrace_arch_code_modify_post_process()
> > set_all_modules_text_ro()
> > klp_write_object_relocations()
> > apply_relocate_add()
> > *patches read-only code* - BOOM
> >
> > A similar race exists when toggling ftrace while loading a livepatch
> > module.
> >
> > Fix it by ensuring that the livepatch and ftrace code patching
> > operations -- and their respective permissions changes -- are protected
> > by the text_mutex.
> >
> > Reported-by: Johannes Erdfelt <johannes@xxxxxxxxxxx>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > ---
> > kernel/livepatch/core.c | 6 ++++++
> > kernel/module.c | 9 +++++++++
> > kernel/trace/ftrace.c | 10 +++++++++-
> > 3 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 91cd519756d3..2d17e6e364b5 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -30,6 +30,7 @@
> > #include <linux/elf.h>
> > #include <linux/moduleloader.h>
> > #include <linux/completion.h>
> > +#include <linux/memory.h>
> > #include <asm/cacheflush.h>
> > #include "core.h"
> > #include "patch.h"
> > @@ -730,16 +731,21 @@ static int klp_init_object_loaded(struct klp_patch *patch,
> > struct klp_func *func;
> > int ret;
> >
> > + mutex_lock(&text_mutex);
> > +
> > module_disable_ro(patch->mod);
> > ret = klp_write_object_relocations(patch->mod, obj);
> > if (ret) {
> > module_enable_ro(patch->mod, true);
> > + mutex_unlock(&text_mutex);
> > return ret;
> > }
> >
> > arch_klp_init_object_loaded(patch, obj);
> > module_enable_ro(patch->mod, true);
> >
> > + mutex_unlock(&text_mutex);
> > +
> > klp_for_each_func(obj, func) {
> > ret = klp_find_object_symbol(obj->name, func->old_name,
> > func->old_sympos,
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 6e6712b3aaf5..4d9f3281c0c5 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -64,6 +64,7 @@
> > #include <linux/bsearch.h>
> > #include <linux/dynamic_debug.h>
> > #include <linux/audit.h>
> > +#include <linux/memory.h>
> > #include <uapi/linux/module.h>
> > #include "module-internal.h"
> >
> > @@ -1943,6 +1944,8 @@ static void frob_writable_data(const struct module_layout *layout,
> > /* livepatching wants to disable read-only so it can frob module. */
> > void module_disable_ro(const struct module *mod)
> > {
> > + lockdep_assert_held(&text_mutex);
> > +
> > if (!rodata_enabled)
> > return;
> >
> > @@ -1955,6 +1958,8 @@ void module_disable_ro(const struct module *mod)
> >
> > void module_enable_ro(const struct module *mod, bool after_init)
> > {
> > + lockdep_assert_held(&text_mutex);
>
> Hi Josh!
>
> The lockdep WARN_ON triggers when loading a module under a lockdep enabled kernel:
>
> [ 6.139583] WARNING: CPU: 0 PID: 102 at /home/ppyu/jeyu-linux/kernel/module.c:1961 module_enable_ro+0x121/0x130
> [ 6.143641] Modules linked in:
> [ 6.144879] CPU: 0 PID: 102 Comm: insmod Not tainted 5.2.0-rc2+ #1
> [ 6.147325] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fc-prebuilt.qemu-project.org 04/01/2014
> [ 6.152046] RIP: 0010:module_enable_ro+0x121/0x130
> [ 6.153986] Code: 48 c7 c6 70 97 05 81 5d 41 5c 41 5d e9 b8 e2 ff ff be ff ff ff ff 48 c7 c7 40 b2 65 82 e8 a7 c7 fa ff 85 c0 0f 85 f8 fe ff ff <0f> 0b e9 f1 fe ff ff 0f 1f 84 00 00 00 00 00 8b 05 9e a5 66 01 55
> [ 6.161277] RSP: 0018:ffffc9000041fd18 EFLAGS: 00010246
> [ 6.162919] RAX: 0000000000000000 RBX: ffffffffc0002000 RCX: 000000000000000b
> [ 6.164783] RDX: ffff88803d793fc0 RSI: ffffffff8265b240 RDI: ffff88803d7947e8
> [ 6.166820] RBP: 0000000000000000 R08: 000000000007ade8 R09: ffffffff823989a6
> [ 6.168579] R10: ffffc9000049c8e8 R11: 0000000000000002 R12: ffffffffc00021c0
> [ 6.169994] R13: 0000000000000000 R14: 0000000000000030 R15: ffffc9000041fe78
> [ 6.171390] FS: 00007fdc49708700(0000) GS:ffff88803d800000(0000) knlGS:0000000000000000
> [ 6.173007] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 6.174047] CR2: 00007fdc496c0000 CR3: 000000003fb84000 CR4: 00000000000006f0
> [ 6.175168] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 6.176326] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 6.177384] Call Trace:
> [ 6.177730] load_module+0x1b96/0x2620
> [ 6.178231] ? rcu_read_lock_sched_held+0x53/0x60
> [ 6.178865] ? __alloc_pages_nodemask+0x2fa/0x350
> [ 6.179497] ? __do_sys_init_module+0x135/0x170
> [ 6.180097] ? _cond_resched+0x10/0x40
> [ 6.180594] __do_sys_init_module+0x135/0x170
> [ 6.181177] do_syscall_64+0x4b/0x1c0
> [ 6.181681] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> It triggers since we call module_enable_ro() along the load_module()
> path to prepare for a coming module. We don't take the text_mutex
> there since we're not modifying any text, just memory protections.
> Leaving the lockdep assert in module_disable_ro() and
> set_all_modules_text_*() should be fine though, since I think
> livepatch and ftrace are the only users of those functions.

Yeah, I discovered that already:

https://lkml.kernel.org/r/20190522130014.yvkbio62meatqvwf@treble

The new patch (which I will hopefully be able to post soon) will fix
that up.

--
Josh