While running stress tests on adding and deleting ftrace instances I hit this bug: BUG: unable to handle kernel NULL pointer dereference at 0000000000000020 IP: [<ffffffff812d8bc5>] selinux_inode_permission+0x85/0x160 PGD 63681067 PUD 7ddbe067 PMD 0 Oops: 0000 [#1] PREEMPT CPU: 0 PID: 5634 Comm: ftrace-test-mki Not tainted 3.13.0-rc4-test-00033-gd2a6dde-dirty #20 Hardware name: /DG965MQ, BIOS MQ96510J.86A.0372.2006.0605.1717 06/05/2006 task: ffff880078375800 ti: ffff88007ddb0000 task.ti: ffff88007ddb0000 RIP: 0010:[<ffffffff812d8bc5>] [<ffffffff812d8bc5>] selinux_inode_permission+0x85/0x160 RSP: 0018:ffff88007ddb1c48 EFLAGS: 00010246 RAX: 0000000000000000 RBX: 0000000000800000 RCX: ffff88006dd43840 RDX: 0000000000000001 RSI: 0000000000000081 RDI: ffff88006ee46000 RBP: ffff88007ddb1c88 R08: 0000000000000000 R09: ffff88007ddb1c54 R10: 6e6576652f6f6f66 R11: 0000000000000003 R12: 0000000000000000 R13: 0000000000000081 R14: ffff88006ee46000 R15: 0000000000000000 FS: 00007f217b5b6700(0000) GS:ffffffff81e21000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033^M CR2: 0000000000000020 CR3: 000000006a0fe000 CR4: 00000000000007f0 Stack: 0000000000000081 ffff88006ee46000 0000000000000081 ffffffff812d8b45 ffff88006ee46000 0000000000000081 ffff880078375800 ffff880078375800 ffff88007ddb1c98 ffffffff812d358c ffff88007ddb1cb8 ffffffff811364f1 Call Trace:^M [<ffffffff812d8b45>] ? selinux_inode_permission+0x5/0x160 [<ffffffff812d358c>] security_inode_permission+0x1c/0x30 [<ffffffff811364f1>] __inode_permission+0x41/0xa0 [<ffffffff81136568>] inode_permission+0x18/0x50 [<ffffffff811378b6>] link_path_walk+0x66/0x920 [<ffffffff810875a5>] ? __rcu_read_lock+0x5/0x20 [<ffffffff8113a9e6>] path_openat+0xa6/0x6c0 [<ffffffff8113b000>] ? path_openat+0x6c0/0x6c0 [<ffffffff810c4e69>] ? __trace_graph_entry+0x49/0xc0 [<ffffffff8112b196>] ? do_sys_open+0x146/0x240 [<ffffffff8113b043>] do_filp_open+0x43/0xa0 [<ffffffff8113b005>] ? do_filp_open+0x5/0xa0 [<ffffffff8112b196>] do_sys_open+0x146/0x240 [<ffffffff8112b2ae>] SyS_open+0x1e/0x20 [<ffffffff81948cd0>] system_call_fastpath+0x16/0x1b Code: 84 a1 00 00 00 81 e3 00 20 00 00 89 d8 83 c8 02 40 f6 c6 04 0f 45 d8 40 f6 c6 08 74 71 80 cf 02 49 8b 46 38 4c 8d 4d cc 45 31 c0 <0f> b7 50 20 8b 70 1c 48 8b 41 70 89 d9 8b 78 04 e8 36 cf ff ff RIP [<ffffffff812d8bc5>] selinux_inode_permission+0x85/0x160 RSP <ffff88007ddb1c48> CR2: 0000000000000020 ---[ end trace 9d800e5ac5059462 ]--- Investigating, I found that the inode->i_security was NULL, and the dereference of it caused the oops. in selinux_inode_permission(): ---- isec = inode->i_security; rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd); ---- Note, the crash came from stressing the deletion and reading of debugfs files. I was not able to recreate this via normal files. But I'm not sure they are safe. It may just be that the race window is much harder to hit. What seems to have happened (and what I have traced), is the file is being opened at the same time the file or directory is being deleted. As the dentry and inode locks are not held during the path walk, nor is the inodes ref counts being incremented, there is nothing saving these structures from being discarded except for an rcu_read_lock(). The rcu_read_lock() protects against freeing of the inode, but it does not protect freeing of the inode_security_struct. Now if the freeing of the i_security happens with a call_rcu(), and the i_security field of the inode is not changed (it gets freed as the inode gets freed) then there will be no issue here. (Linus Torvalds suggested not setting the field to NULL such that we do not need to check if it is NULL in the permission check). Note, this is a hack, but it fixes the problem at hand. A real fix is to restructure the destroy_inode() to call all the destructor handlers from the RCU callback. But that is a major job to do, and requires a lot of work. For now, we just band-aid this bug with this fix (it works), and work on a more maintainable solution in the future. Link: http://lkml.kernel.org/r/20140109101932.0508dec7@xxxxxxxxxxxxxxxxxx Link: http://lkml.kernel.org/r/20140109182756.17abaaa8@xxxxxxxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> --- security/selinux/hooks.c | 20 ++++++++++++++++++-- security/selinux/include/objsec.h | 5 ++++- 2 files changed, 22 insertions(+), 3 deletions(-) diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 419491d..782045d 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -234,6 +234,14 @@ static int inode_alloc_security(struct inode *inode) return 0; } +static void inode_free_rcu(struct rcu_head *head) +{ + struct inode_security_struct *isec; + + isec = container_of(head, struct inode_security_struct, rcu); + kmem_cache_free(sel_inode_cache, isec); +} + static void inode_free_security(struct inode *inode) { struct inode_security_struct *isec = inode->i_security; @@ -244,8 +252,16 @@ static void inode_free_security(struct inode *inode) list_del_init(&isec->list); spin_unlock(&sbsec->isec_lock); - inode->i_security = NULL; - kmem_cache_free(sel_inode_cache, isec); + /* + * The inode may still be referenced in a path walk and + * a call to selinux_inode_permission() can be made + * after inode_free_security() is called. Ideally, the VFS + * wouldn't do this, but fixing that is a much harder + * job. For now, simply free the i_security via RCU, and + * leave the current inode->i_security pointer intact. + * The inode will be freed after the RCU grace period too. + */ + call_rcu(&isec->rcu, inode_free_rcu); } static int file_alloc_security(struct file *file) diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h index b1dfe10..078e553 100644 --- a/security/selinux/include/objsec.h +++ b/security/selinux/include/objsec.h @@ -38,7 +38,10 @@ struct task_security_struct { struct inode_security_struct { struct inode *inode; /* back pointer to inode object */ - struct list_head list; /* list of inode_security_struct */ + union { + struct list_head list; /* list of inode_security_struct */ + struct rcu_head rcu; /* for freeing the inode_security_struct */ + }; u32 task_sid; /* SID of creating task */ u32 sid; /* SID of this object */ u16 sclass; /* security class of this object */ -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
