linux-kernel - Re: [PATCH] selinux: use GFP_ATOMIC in convert

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Date:   Tue, 18 Oct 2022 15:58:17 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     "GONG, Ruiqi" <gongruiqi1@...wei.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiu Jianfeng <xiujianfeng@...wei.com>
Subject: Re: [PATCH] selinux: use GFP_ATOMIC in convert_context()

On Tue, Oct 18, 2022 at 8:46 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> On Tue, Oct 18, 2022 at 2:01 PM GONG, Ruiqi <gongruiqi1@...wei.com> wrote:
> > The following BUG_ON was triggered on a hardware environment:
> >
> >   SELinux: Converting 162 SID table entries...
> >   BUG: sleeping function called from invalid context at __might_sleep_rtos+0x60/0x74 0x0
> >   in_atomic(): 1, irqs_disabled(): 128, non_block: 0, pid: 5943, name: tar
> >   CPU: 7 PID: 5943 Comm: tar Tainted: P O 5.10.0 #1
> >   Call trace:
> >    dump_backtrace+0x0/0x1c8
> >    show_stack+0x18/0x28
> >    dump_stack+0xe8/0x15c
> >    ___might_sleep_rtos+0x168/0x17c
> >    __might_sleep_rtos+0x60/0x74
> >    __kmalloc_track_caller+0xa0/0x7dc
> >    kstrdup+0x54/0xac
> >    convert_context+0x48/0x2e4
> >    sidtab_context_to_sid+0x1c4/0x36c
> >    security_context_to_sid_core+0x168/0x238
> >    security_context_to_sid_default+0x14/0x24
> >    inode_doinit_use_xattr+0x164/0x1e4
> >    inode_doinit_with_dentry+0x1c0/0x488
> >    selinux_d_instantiate+0x20/0x34
> >    security_d_instantiate+0x70/0xbc
> >    d_splice_alias+0x4c/0x3c0
> >    ext4_lookup+0x1d8/0x200 [ext4]
> >    __lookup_slow+0x12c/0x1e4
> >    walk_component+0x100/0x200
> >    path_lookupat+0x88/0x118
> >    filename_lookup+0x98/0x130
> >    user_path_at_empty+0x48/0x60
> >    vfs_statx+0x84/0x140
> >    vfs_fstatat+0x20/0x30
> >    __se_sys_newfstatat+0x30/0x74
> >    __arm64_sys_newfstatat+0x1c/0x2c
> >    el0_svc_common.constprop.0+0x100/0x184
> >    do_el0_svc+0x1c/0x2c
> >    el0_svc+0x20/0x34
> >    el0_sync_handler+0x80/0x17c
> >    el0_sync+0x13c/0x140
> >   SELinux: Context system_u:object_r:pssp_rsyslog_log_t:s0:c0 is not valid (left unmapped).
> >
> > It was found that convert_context() (hooked by convert->func) might
> > sleep in a critial section of spin_lock_irqsave in
> > sidtab_context_to_sid(). Fix this problem by changing the memory
> > allocation in convert_context() from GFP_KERNEL to GFP_ATOMIC.
>
> Good catch! However, convert_context() (and
> sidtab_convert_params::func) has two callers:
> 1. sidtab_context_to_sid(), which requires GFP_ATOMIC,
> 2. sidtab_convert_tree()/sidtab_convert(), where GFP_KERNEL would be okay.
>
> So a more optimal fix would be to add a gfp_t argument to
> convert_context()/sidtab_convert_params::func and pass
> GFP_KERNEL/_ATOMIC as appropriate in the individual callers.

Agreed.  Please make the changes Ondrej is suggesting.

-- 
paul-moore.com