linux-kernel - [PATCH v2 1/2] futex: futex_wake

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Date:   Mon, 23 Oct 2017 13:41:51 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     mingo@...hat.com
Cc:     linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH v2 1/2] futex: futex_wake_op, do not fail on invalid op

In 30d6e0a4190d ("futex: Remove duplicated code and fix undefined
behaviour"), I let FUTEX_WAKE_OP to fail on invalid op. Namely when
op should be considered as shift and the shift is out of range (< 0 or
> 31).

But strace's test suite does this madness:
futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xa0caffee);
futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xbadfaced);
futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xffffffff);

When I pick the first 0xa0caffee, it decodes as:
0x80000000 & 0xa0caffee: oparg is shift
0x70000000 & 0xa0caffee: op is FUTEX_OP_OR
0x0f000000 & 0xa0caffee: cmp is FUTEX_OP_CMP_EQ
0x00fff000 & 0xa0caffee: oparg is sign-extended 0xcaf = -849
0x00000fff & 0xa0caffee: cmparg is sign-extended 0xfee = -18

That means the op tries to do this:
  (futex |= (1 << (-849))) == -18
which is completely bogus. The new check of op in the code is:
        if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
                if (oparg < 0 || oparg > 31)
                        return -EINVAL;
                oparg = 1 << oparg;
        }

which results obviously in the "Invalid argument" errno:
----8<--------8<--------8<--------8<--------8<----
FAIL: futex
===========

futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xa0caffee) = -1: Invalid argument
futex.test: failed test: ../futex failed with code 1
----8<--------8<--------8<--------8<--------8<----

So let us soften the failure to print only a (ratelimited) message,
crop the value and continue as if it were right. When userspace keeps
up, we can switch this to return -EINVAL again.

[v2]
Do not return 0 immediatelly, proceed with the cropped value.

Fixes: 30d6e0a4190d ("futex: Remove duplicated code and fix undefined behaviour")
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Darren Hart <dvhart@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
 kernel/futex.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0518a0bfc746..0d638f008bb1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1570,8 +1570,16 @@ static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
 	int oldval, ret;
 
 	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
-		if (oparg < 0 || oparg > 31)
-			return -EINVAL;
+		if (oparg < 0 || oparg > 31) {
+			char comm[sizeof(current->comm)];
+			/*
+			 * kill this print and return -EINVAL when userspace
+			 * is sane again
+			 */
+			pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
+					get_task_comm(comm, current), oparg);
+			oparg &= 31;
+		}
 		oparg = 1 << oparg;
 	}
 
-- 
2.14.2