smack: fix possible use after frees in task_security() callers

We hit use after free on dereferncing pointer to task_smack struct in
smk_of_task() called from smack_task_to_inode().

task_security() macro uses task_cred_xxx() to get pointer to the task_smack.
task_cred_xxx() could be used only for non-pointer members of task's
credentials. It cannot be used for pointer members since what they point
to may disapper after dropping RCU read lock.

Mainly task_security() used this way:
	smk_of_task(task_security(p))

Intead of this introduce function smk_of_task_struct() which
takes task_struct as argument and returns pointer to smk_known struct
and do this under RCU read lock.
Bogus task_security() macro is not used anymore, so remove it.

KASan's report for this:

	AddressSanitizer: use after free in smack_task_to_inode+0x50/0x70 at addr c4635600
	=============================================================================
	BUG kmalloc-64 (Tainted: PO): kasan error
	-----------------------------------------------------------------------------

	Disabling lock debugging due to kernel taint
	INFO: Allocated in new_task_smack+0x44/0xd8 age=39 cpu=0 pid=1866
		kmem_cache_alloc_trace+0x88/0x1bc
		new_task_smack+0x44/0xd8
		smack_cred_prepare+0x48/0x21c
		security_prepare_creds+0x44/0x4c
		prepare_creds+0xdc/0x110
		smack_setprocattr+0x104/0x150
		security_setprocattr+0x4c/0x54
		proc_pid_attr_write+0x12c/0x194
		vfs_write+0x1b0/0x370
		SyS_write+0x5c/0x94
		ret_fast_syscall+0x0/0x48
	INFO: Freed in smack_cred_free+0xc4/0xd0 age=27 cpu=0 pid=1564
		kfree+0x270/0x290
		smack_cred_free+0xc4/0xd0
		security_cred_free+0x34/0x3c
		put_cred_rcu+0x58/0xcc
		rcu_process_callbacks+0x738/0x998
		__do_softirq+0x264/0x4cc
		do_softirq+0x94/0xf4
		irq_exit+0xbc/0x120
		handle_IRQ+0x104/0x134
		gic_handle_irq+0x70/0xac
		__irq_svc+0x44/0x78
		_raw_spin_unlock+0x18/0x48
		sync_inodes_sb+0x17c/0x1d8
		sync_filesystem+0xac/0xfc
		vdfs_file_fsync+0x90/0xc0
		vfs_fsync_range+0x74/0x7c
	INFO: Slab 0xd3b23f50 objects=32 used=31 fp=0xc4635600 flags=0x4080
	INFO: Object 0xc4635600 @offset=5632 fp=0x  (null)

	Bytes b4 c46355f0: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
	Object c4635600: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
	Object c4635610: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
	Object c4635620: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
	Object c4635630: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
	Redzone c4635640: bb bb bb bb                                      ....
	Padding c46356e8: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
	Padding c46356f8: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
	CPU: 5 PID: 834 Comm: launchpad_prelo Tainted: PBO 3.10.30 #1
	Backtrace:
	[<c00233a4>] (dump_backtrace+0x0/0x158) from [<c0023dec>] (show_stack+0x20/0x24)
	 r7:c4634010 r6:d3b23f50 r5:c4635600 r4:d1002140
	[<c0023dcc>] (show_stack+0x0/0x24) from [<c06d6d7c>] (dump_stack+0x20/0x28)
	[<c06d6d5c>] (dump_stack+0x0/0x28) from [<c01c1d50>] (print_trailer+0x124/0x144)
	[<c01c1c2c>] (print_trailer+0x0/0x144) from [<c01c1e88>] (object_err+0x3c/0x44)
	 r7:c4635600 r6:d1002140 r5:d3b23f50 r4:c4635600
	[<c01c1e4c>] (object_err+0x0/0x44) from [<c01cac18>] (kasan_report_error+0x2b8/0x538)
	 r6:d1002140 r5:d3b23f50 r4:c6429cf8 r3:c09e1aa7
	[<c01ca960>] (kasan_report_error+0x0/0x538) from [<c01c9430>] (__asan_load4+0xd4/0xf8)
	[<c01c935c>] (__asan_load4+0x0/0xf8) from [<c031e168>] (smack_task_to_inode+0x50/0x70)
	 r5:c4635600 r4:ca9da000
	[<c031e118>] (smack_task_to_inode+0x0/0x70) from [<c031af64>] (security_task_to_inode+0x3c/0x44)
	 r5:cca25e80 r4:c0ba9780
	[<c031af28>] (security_task_to_inode+0x0/0x44) from [<c023d614>] (pid_revalidate+0x124/0x178)
	 r6:00000000 r5:cca25e80 r4:cbabe3c0 r3:00008124
	[<c023d4f0>] (pid_revalidate+0x0/0x178) from [<c01db98c>] (lookup_fast+0x35c/0x43y4)
	 r9:c6429efc r8:00000101 r7:c079d940 r6:c6429e90 r5:c6429ed8 r4:c83c4148
	[<c01db630>] (lookup_fast+0x0/0x434) from [<c01deec8>] (do_last.isra.24+0x1c0/0x1108)
	[<c01ded08>] (do_last.isra.24+0x0/0x1108) from [<c01dff04>] (path_openat.isra.25+0xf4/0x648)
	[<c01dfe10>] (path_openat.isra.25+0x0/0x648) from [<c01e1458>] (do_filp_open+0x3c/0x88)
	[<c01e141c>] (do_filp_open+0x0/0x88) from [<c01ccb28>] (do_sys_open+0xf0/0x198)
	 r7:00000001 r6:c0ea2180 r5:0000000b r4:00000000
	[<c01cca38>] (do_sys_open+0x0/0x198) from [<c01ccc00>] (SyS_open+0x30/0x34)
	[<c01ccbd0>] (SyS_open+0x0/0x34) from [<c001db80>] (ret_fast_syscall+0x0/0x48)
	Read of size 4 by thread T834:
	Memory state around the buggy address:
	 c4635380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
	 c4635400: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
	 c4635480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
	 c4635500: 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc
	 c4635580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
	>c4635600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
	           ^
	 c4635680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
	 c4635700: 00 00 00 00 04 fc fc fc fc fc fc fc fc fc fc fc
	 c4635780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
	 c4635800: 00 00 00 00 00 00 04 fc fc fc fc fc fc fc fc fc
	 c4635880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
	==================================================================
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Cc: <stable@vger.kernel.org>

security/smack/smack.h

@@ -299,6 +299,16 @@ static inline struct smack_known *smk_of_task(const struct task_smack *tsp)
 	return tsp->smk_task;
 }
 
+static inline struct smack_known *smk_of_task_struct(const struct task_struct *t)
+{
+	struct smack_known *skp;
+
+	rcu_read_lock();
+	skp = smk_of_task(__task_cred(t)->security);
+	rcu_read_unlock();
+	return skp;
+}
+
 /*
  * Present a pointer to the forked smack label entry in an task blob.
  */

security/smack/smack_lsm.c

@@ -43,8 +43,6 @@
 #include <linux/binfmts.h>
 #include "smack.h"
 
-#define task_security(task)	(task_cred_xxx((task), security))
-
 #define TRANS_TRUE	"TRUE"
 #define TRANS_TRUE_SIZE	4
 
@@ -123,7 +121,7 @@ static int smk_bu_current(char *note, struct smack_known *oskp,
 static int smk_bu_task(struct task_struct *otp, int mode, int rc)
 {
 	struct task_smack *tsp = current_security();
-	struct task_smack *otsp = task_security(otp);
+	struct smack_known *smk_task = smk_of_task_struct(otp);
 	char acc[SMK_NUM_ACCESS_TYPE + 1];
 
 	if (rc <= 0)
@@ -131,7 +129,7 @@ static int smk_bu_task(struct task_struct *otp, int mode, int rc)
 
 	smk_bu_mode(mode, acc);
 	pr_info("Smack Bringup: (%s %s %s) %s to %s\n",
-		tsp->smk_task->smk_known, otsp->smk_task->smk_known, acc,
+		tsp->smk_task->smk_known, smk_task->smk_known, acc,
 		current->comm, otp->comm);
 	return 0;
 }
@@ -352,7 +350,8 @@ static int smk_ptrace_rule_check(struct task_struct *tracer,
 		saip = &ad;
 	}
 
-	tsp = task_security(tracer);
+	rcu_read_lock();
+	tsp = __task_cred(tracer)->security;
 	tracer_known = smk_of_task(tsp);
 
 	if ((mode & PTRACE_MODE_ATTACH) &&
@@ -372,11 +371,14 @@ static int smk_ptrace_rule_check(struct task_struct *tracer,
 				  tracee_known->smk_known,
 				  0, rc, saip);
 
+		rcu_read_unlock();
 		return rc;
 	}
 
 	/* In case of rule==SMACK_PTRACE_DEFAULT or mode==PTRACE_MODE_READ */
 	rc = smk_tskacc(tsp, tracee_known, smk_ptrace_mode(mode), saip);
+
+	rcu_read_unlock();
 	return rc;
 }
 
@@ -403,7 +405,7 @@ static int smack_ptrace_access_check(struct task_struct *ctp, unsigned int mode)
 	if (rc != 0)
 		return rc;
 
-	skp = smk_of_task(task_security(ctp));
+	skp = smk_of_task_struct(ctp);
 
 	rc = smk_ptrace_rule_check(current, skp, mode, __func__);
 	return rc;
@@ -1830,7 +1832,7 @@ static int smk_curacc_on_task(struct task_struct *p, int access,
 				const char *caller)
 {
 	struct smk_audit_info ad;
-	struct smack_known *skp = smk_of_task(task_security(p));
+	struct smack_known *skp = smk_of_task_struct(p);
 	int rc;
 
 	smk_ad_init(&ad, caller, LSM_AUDIT_DATA_TASK);
@@ -1883,7 +1885,7 @@ static int smack_task_getsid(struct task_struct *p)
  */
 static void smack_task_getsecid(struct task_struct *p, u32 *secid)
 {
-	struct smack_known *skp = smk_of_task(task_security(p));
+	struct smack_known *skp = smk_of_task_struct(p);
 
 	*secid = skp->smk_secid;
 }
@@ -1990,7 +1992,7 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 {
 	struct smk_audit_info ad;
 	struct smack_known *skp;
-	struct smack_known *tkp = smk_of_task(task_security(p));
+	struct smack_known *tkp = smk_of_task_struct(p);
 	int rc;
 
 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_TASK);
@@ -2044,7 +2046,7 @@ static int smack_task_wait(struct task_struct *p)
 static void smack_task_to_inode(struct task_struct *p, struct inode *inode)
 {
 	struct inode_smack *isp = inode->i_security;
-	struct smack_known *skp = smk_of_task(task_security(p));
+	struct smack_known *skp = smk_of_task_struct(p);
 
 	isp->smk_inode = skp;
 }
@@ -3226,7 +3228,7 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
  */
 static int smack_getprocattr(struct task_struct *p, char *name, char **value)
 {
-	struct smack_known *skp = smk_of_task(task_security(p));
+	struct smack_known *skp = smk_of_task_struct(p);
 	char *cp;
 	int slen;