[BUG] security_release_secctx seems broken

From: Konstantin Khlebnikov
Date: Sat Sep 16 2017 - 14:19:06 EST

I've got this kmemleak splat

unreferenced object 0xffff880f687ff6a8 (size 32):
comm "cp", pid 4279, jiffies 4295784487 (age 2866.296s)
hex dump (first 32 bytes):
01 00 00 02 02 00 08 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 .....kkkkkkkkkk.
[<ffffffff8192d77a>] kmemleak_alloc+0x4a/0xa0
[<ffffffff8125b721>] __kmalloc_track_caller+0x171/0x280
[<ffffffff812125c7>] krealloc+0xa7/0xc0
[<ffffffff812abb2d>] vfs_getxattr_alloc+0xbd/0x110
[<ffffffff813ba789>] cap_inode_getsecurity+0x79/0x200
[<ffffffff813be4d2>] security_inode_getsecurity+0x52/0x80
[<ffffffff812aad1a>] xattr_getsecurity+0x3a/0xa0
[<ffffffff812aadf4>] vfs_getxattr+0x74/0xa0
[<ffffffff812ab27e>] getxattr+0x9e/0x170
[<ffffffff812abd2a>] SyS_fgetxattr+0x5a/0x90
[<ffffffff8193a67b>] entry_SYSCALL_64_fastpath+0x1e/0xae
[<ffffffffffffffff>] 0xffffffffffffffff

allocated in xattr_getsecurity() security_inode_getsecurity() -> cap_inode_getsecurity()
should be freed by security_release_secctx() but there is no release_secctx hook.

I don't know details about modern security models stack but it
seems security_release_secctx() have to know which layer owns
secdata to free it property: selinux just calls kfree,
capability_hooks should do the same, but other modules returns
non-freeable pointers. Currently security_release_secctx() calls
release_secctx for all models, this is obviously wrong.