[tip:locking/urgent] futex: Ensure that futex address is aligned in handle_futex_death()

[tip-bot for Chen Jie]

Commit-ID:  5a07168d8d89b00fe1760120714378175b3ef992
Gitweb:     https://git.kernel.org/tip/5a07168d8d89b00fe1760120714378175b3ef992
Author:     Chen Jie <chenjie6@xxxxxxxxxx>
AuthorDate: Fri, 15 Mar 2019 03:44:38 +0000
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 22 Mar 2019 13:05:26 +0100

futex: Ensure that futex address is aligned in handle_futex_death()

The futex code requires that the user space addresses of futexes are 32bit
aligned. sys_futex() checks this in futex_get_keys() but the robust list
code has no alignment check in place.

As a consequence the kernel crashes on architectures with strict alignment
requirements in handle_futex_death() when trying to cmpxchg() on an
unaligned futex address which was retrieved from the robust list.

[ tglx: Rewrote changelog, proper sizeof() based alignement check and add
  	comment ]

Fixes: 0771dfefc9e5 ("[PATCH] lightweight robust futexes: core")
Signed-off-by: Chen Jie <chenjie6@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: <dvhart@xxxxxxxxxxxxx>
Cc: <peterz@xxxxxxxxxxxxx>
Cc: <zengweilin@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/1552621478-119787-1-git-send-email-chenjie6@xxxxxxxxxx

---
 kernel/futex.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index c3b73b0311bc..9e40cf7be606 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3436,6 +3436,10 @@ static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int p
 {
 	u32 uval, uninitialized_var(nval), mval;
 
+	/* Futex address must be 32bit aligned */
+	if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
+		return -1;
+
 retry:
 	if (get_user(uval, uaddr))
 		return -1;