watch_queue: Actually free the watch

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: watch_queue: Actually free the watch free_watch() does everything barring actually freeing the watch object. Fix this by adding the missing kfree. kmemleak produces a report something like the following. Note that as an address can be seen in the first word, the watch would appear to have gone through call_rcu(). BUG: memory leak unreferenced object 0xffff88810ce4a200 (size 96): comm "syz-executor352", pid 3605, jiffies 4294947473 (age 13.720s) hex dump (first 32 bytes): e0 82 48 0d 81 88 ff ff 00 00 00 00 00 00 00 00 ..H............. 80 a2 e4 0c 81 88 ff ff 00 00 00 00 00 00 00 00 ................ backtrace: [<ffffffff8214e6cc>] kmalloc include/linux/slab.h:581 [inline] [<ffffffff8214e6cc>] kzalloc include/linux/slab.h:714 [inline] [<ffffffff8214e6cc>] keyctl_watch_key+0xec/0x2e0 security/keys/keyctl.c:1800 [<ffffffff8214ec84>] __do_sys_keyctl+0x3c4/0x490 security/keys/keyctl.c:2016 [<ffffffff84493a25>] do_syscall_x64 arch/x86/entry/common.c:50 [inline] [<ffffffff84493a25>] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 [<ffffffff84600068>] entry_SYSCALL_64_after_hwframe+0x44/0xae

References

• https://git.kernel.org/stable/c/9d92be1a09fbb3dd65600dbfe7eedb40e7228e4b
• https://git.kernel.org/stable/c/f69aecb49968e14196366bbe896eab0a904229f5
• https://git.kernel.org/stable/c/7e8c9b0df07a77f0d072603b8ced2677e30e1893
• https://git.kernel.org/stable/c/31824613a42aacdcbeb325bf07a1c8247a11ebe2
• https://git.kernel.org/stable/c/3d8dcf278b1ee1eff1e90be848fa2237db4c07a7