Re: [PATCH v2 4/7] fs/proc/task_mmu.c: shift mm_access() from m_start() to proc_maps_open()

[ebiederm@xmission.com (Eric W. Biederman)]

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Tue, Aug 05, 2014 at 09:46:55PM +0200, Oleg Nesterov wrote:
>> A simple test-case from Kirill Shutemov
>> 
>> 	cat /proc/self/maps >/dev/null
>> 	chmod +x /proc/self/net/packet
>> 	exec /proc/self/net/packet
>> 
>> makes lockdep unhappy, cat/exec take seq_file->lock + cred_guard_mutex in
>> the opposite order.
>
> Oleg, I see it again with almost the same test-case:
>
> 	cat /proc/self/stack >/dev/null
> 	chmod +x /proc/self/net/packet
> 	exec /proc/self/net/packet
>
> Looks like bunch of proc files were converted to use seq_file by Alexey
> Dobriyan around the same time you've fixed the issue for /proc/pid/maps.
>
> More generic test-case:
>
> 	find /proc/self/ -type f -exec dd if='{}' of=/dev/null bs=1 count=1 ';' 2>/dev/null
> 	chmod +x /proc/self/net/packet
> 	exec /proc/self/net/packet
>
> David, any justification for allowing chmod +x for files under
> /proc/pid/net?

I don't think there are any good reasons for allowing chmod +x for the
proc generic files.   Certainly executing any of them is nonsense.

I do recall some weird conner cases existing.  I think they resulted
in a need to preserve chmod if not chmod +x.  This is just me saying
tread carefully before you change anything.

It really should be safe to tweak proc_notify_change to not allow
messing with the executable bits of proc files.

> [    2.042212] ======================================================
> [    2.042930] [ INFO: possible circular locking dependency detected ]
> [    2.043648] 3.18.0-rc7-00003-g3a18ca061311-dirty #237 Not tainted
> [    2.044350] -------------------------------------------------------
> [    2.045054] sh/94 is trying to acquire lock:
> [    2.045546]  (&p->lock){+.+.+.}, at: [<ffffffff811e12fd>] seq_read+0x3d/0x3e0
> [    2.045781] 
> [    2.045781] but task is already holding lock:
> [    2.045781]  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff811c0e3d>] prepare_bprm_creds+0x2d/0x90
> [    2.045781] 
> [    2.045781] which lock already depends on the new lock.
> [    2.045781] 
> [    2.045781] 
> [    2.045781] the existing dependency chain (in reverse order) is:
> [    2.045781] 
> -> #1 (&sig->cred_guard_mutex){+.+.+.}:
> [    2.045781]        [<ffffffff810a6e99>] __lock_acquire+0x4d9/0xd40
> [    2.045781]        [<ffffffff810a7ff2>] lock_acquire+0xd2/0x2a0
> [    2.045781]        [<ffffffff81849da6>] mutex_lock_killable_nested+0x66/0x460
> [    2.045781]        [<ffffffff81229de4>] lock_trace+0x24/0x70
> [    2.045781]        [<ffffffff81229e8f>] proc_pid_stack+0x5f/0xe0
> [    2.045781]        [<ffffffff81227244>] proc_single_show+0x54/0xa0
> [    2.045781]        [<ffffffff811e13a0>] seq_read+0xe0/0x3e0
> [    2.045781]        [<ffffffff811b9377>] vfs_read+0x97/0x180
> [    2.045781]        [<ffffffff811b9f5d>] SyS_read+0x4d/0xc0
> [    2.045781]        [<ffffffff8184e492>] system_call_fastpath+0x12/0x17
> [    2.045781] 
> -> #0 (&p->lock){+.+.+.}:
> [    2.045781]        [<ffffffff810a389f>] validate_chain.isra.36+0xfff/0x1400
> [    2.045781]        [<ffffffff810a6e99>] __lock_acquire+0x4d9/0xd40
> [    2.045781]        [<ffffffff810a7ff2>] lock_acquire+0xd2/0x2a0
> [    2.045781]        [<ffffffff81849629>] mutex_lock_nested+0x69/0x3c0
> [    2.045781]        [<ffffffff811e12fd>] seq_read+0x3d/0x3e0
> [    2.045781]        [<ffffffff81226428>] proc_reg_read+0x48/0x70
> [    2.045781]        [<ffffffff811b9377>] vfs_read+0x97/0x180
> [    2.045781]        [<ffffffff811bf1a8>] kernel_read+0x48/0x60
> [    2.045781]        [<ffffffff811bfb2c>] prepare_binprm+0xdc/0x180
> [    2.045781]        [<ffffffff811c139a>] do_execve_common.isra.29+0x4fa/0x960
> [    2.045781]        [<ffffffff811c1818>] do_execve+0x18/0x20
> [    2.045781]        [<ffffffff811c1b05>] SyS_execve+0x25/0x30
> [    2.045781]        [<ffffffff8184ea49>] stub_execve+0x69/0xa0
> [    2.045781] 
> [    2.045781] other info that might help us debug this:
> [    2.045781] 
> [    2.045781]  Possible unsafe locking scenario:
> [    2.045781] 
> [    2.045781]        CPU0                    CPU1
> [    2.045781]        ----                    ----
> [    2.045781]   lock(&sig->cred_guard_mutex);
> [    2.045781]                                lock(&p->lock);
> [    2.045781]                                lock(&sig->cred_guard_mutex);
> [    2.045781]   lock(&p->lock);
> [    2.045781] 
> [    2.045781]  *** DEADLOCK ***
> [    2.045781] 
> [    2.045781] 1 lock held by sh/94:
> [    2.045781]  #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff811c0e3d>] prepare_bprm_creds+0x2d/0x90
> [    2.045781] 
> [    2.045781] stack backtrace:
> [    2.045781] CPU: 0 PID: 94 Comm: sh Not tainted 3.18.0-rc7-00003-g3a18ca061311-dirty #237
> [    2.045781] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
> [    2.045781]  ffffffff82a48d50 ffff88085427bad8 ffffffff81844a85 0000000000000cac
> [    2.045781]  ffffffff82a654a0 ffff88085427bb28 ffffffff810a1b03 0000000000000000
> [    2.045781]  ffff88085427bb68 ffff88085427bb28 ffff8808547f1500 ffff8808547f1c40
> [    2.045781] Call Trace:
> [    2.045781]  [<ffffffff81844a85>] dump_stack+0x4e/0x68
> [    2.045781]  [<ffffffff810a1b03>] print_circular_bug+0x203/0x310
> [    2.045781]  [<ffffffff810a389f>] validate_chain.isra.36+0xfff/0x1400
> [    2.045781]  [<ffffffff8108fa76>] ? local_clock+0x16/0x30
> [    2.045781]  [<ffffffff810a6e99>] __lock_acquire+0x4d9/0xd40
> [    2.045781]  [<ffffffff810a7ff2>] lock_acquire+0xd2/0x2a0
> [    2.045781]  [<ffffffff811e12fd>] ? seq_read+0x3d/0x3e0
> [    2.045781]  [<ffffffff81849629>] mutex_lock_nested+0x69/0x3c0
> [    2.045781]  [<ffffffff811e12fd>] ? seq_read+0x3d/0x3e0
> [    2.045781]  [<ffffffff8108f9f8>] ? sched_clock_cpu+0x98/0xc0
> [    2.045781]  [<ffffffff811e12fd>] ? seq_read+0x3d/0x3e0
> [    2.045781]  [<ffffffff814050b9>] ? lockref_put_or_lock+0x29/0x40
> [    2.045781]  [<ffffffff811e12fd>] seq_read+0x3d/0x3e0
> [    2.045781]  [<ffffffff814050b9>] ? lockref_put_or_lock+0x29/0x40
> [    2.045781]  [<ffffffff81226428>] proc_reg_read+0x48/0x70
> [    2.045781]  [<ffffffff811b9377>] vfs_read+0x97/0x180
> [    2.045781]  [<ffffffff811bf1a8>] kernel_read+0x48/0x60
> [    2.045781]  [<ffffffff811bfb2c>] prepare_binprm+0xdc/0x180
> [    2.045781]  [<ffffffff811c139a>] do_execve_common.isra.29+0x4fa/0x960
> [    2.092142] tsc: Refined TSC clocksource calibration: 2693.484 MHz
> [    2.045781]  [<ffffffff811c0fd3>] ? do_execve_common.isra.29+0x133/0x960
> [    2.045781]  [<ffffffff8184f04d>] ? retint_swapgs+0xe/0x13
> [    2.045781]  [<ffffffff811c1818>] do_execve+0x18/0x20
> [    2.045781]  [<ffffffff811c1b05>] SyS_execve+0x25/0x30
> [    2.045781]  [<ffffffff8184ea49>] stub_execve+0x69/0xa0