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Date:   Thu, 21 May 2020 14:54:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        arjan@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 0/9] Function Granular KASLR

On Thu, May 21, 2020 at 09:56:31AM -0700, Kristen Carlson Accardi wrote:
> Changes in v2:
> --------------
> * Fix to address i386 build failure
> * Allow module reordering patch to be configured separately so that
>   arm (or other non-x86_64 arches) can take advantage of module function
>   reordering. This support has not be tested by me, but smoke tested by
>   Ard Biesheuvel <ardb@...nel.org> on arm.
> * Fix build issue when building on arm as reported by
>   Ard Biesheuvel <ardb@...nel.org> 
> * minor chages for certain checkpatch warnings and review feedback.

I successfully built and booted this on top of linux-next. For my builds
I include:

CONFIG_LOCK_DEBUGGING_SUPPORT=y
CONFIG_PROVE_LOCKING=y
CONFIG_DEBUG_RT_MUTEXES=y
CONFIG_DEBUG_SPINLOCK=y
CONFIG_DEBUG_MUTEXES=y
CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y
CONFIG_DEBUG_RWSEMS=y
CONFIG_DEBUG_LOCK_ALLOC=y
CONFIG_LOCKDEP=y
CONFIG_DEBUG_ATOMIC_SLEEP=y

which catches various things. One of those (I assume either CONFIG_LOCKDEP
or CONFIG_DEBUG_MUTEXES) has found an issue with kallsyms:

[   34.112989] ------------[ cut here ]------------
[   34.113560] WARNING: CPU: 1 PID: 1997 at kernel/module.c:260 module_assert_mutex+0x29/0x30
[   34.114479] Modules linked in:
[   34.114831] CPU: 1 PID: 1997 Comm: grep Tainted: G        W 5.7.0-rc6-next-20200519+ #497
...
[   34.128556] Call Trace:
[   34.128867]  module_kallsyms_on_each_symbol+0x1d/0xa0
[   34.130238]  kallsyms_on_each_symbol+0xbd/0xd0
[   34.131642]  kallsyms_sorted_open+0x3f/0x70
[   34.132160]  proc_reg_open+0x99/0x180
[   34.133222]  do_dentry_open+0x176/0x400
[   34.134182]  vfs_open+0x2d/0x30
[   34.134579]  do_open.isra.0+0x2a0/0x410
[   34.135058]  path_openat+0x175/0x620
[   34.135506]  do_filp_open+0x91/0x100
[   34.136912]  do_sys_openat2+0x210/0x2d0
[   34.137388]  do_sys_open+0x46/0x80
[   34.137818]  __x64_sys_openat+0x20/0x30
[   34.138288]  do_syscall_64+0x55/0x1d0
[   34.138720]  entry_SYSCALL_64_after_hwframe+0x49/0xb3

Triggering it is easy, just "cat /proc/kallsyms" (and I'd note that I
don't even have any modules loaded). Tracking this down, it just looks
like kallsyms needs to hold a lock while sorting:


diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 558963b275ec..182b16a6079b 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -772,7 +772,9 @@ static int kallsyms_sorted_open(struct inode *inode, struct file *file)
 
 	INIT_LIST_HEAD(list);
 
+	mutex_lock(&module_mutex);
 	ret = kallsyms_on_each_symbol(get_all_symbol_name, list);
+	mutex_unlock(&module_mutex);
 	if (ret != 0)
 		return ret;
 

This fixes it for me. Everything else seems to be lovely. :) Nice work!

-- 
Kees Cook

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