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Subject: [PATCH] futex: send SIGBUS if argument is not aligned on a four-byte
 boundary
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>
Cc:     Maxim Samoylov <max7255@yandex-team.ru>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-api@vger.kernel.org
Date:   Fri, 15 May 2020 18:36:47 +0300
Message-ID: <158955700764.647498.18025770126733698386.stgit@buzz>
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Userspace implementations of mutexes (including glibc) in some cases
retries operation without checking error code from syscall futex.
This is good for performance because most errors are impossible when
locking code trusts itself.

Some errors which could came from outer code are handled automatically,
for example invalid address triggers SIGSEGV on atomic fast path.

But one case turns into nasty busy-loop: when address is unaligned.
futex(FUTEX_WAIT) returns EINVAL immediately and loop goes to retry.

Example which loops inside second call rather than hung peacefully:

#include <stdlib.h>
#include <pthread.h>

int main(int argc, char **argv)
{
	char buf[sizeof(pthread_mutex_t) + 1];
	pthread_mutex_t *mutex = (pthread_mutex_t *)(buf + 1);

	pthread_mutex_init(mutex, NULL);
	pthread_mutex_lock(mutex);
	pthread_mutex_lock(mutex);
}

It seems there is no practical usage for calling syscall futex for
unaligned address. This may be only bug in user space. Let's help
and handle this gracefully without adding extra code on fast path.

This patch sends SIGBUS signal to slay task and break busy-loop.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reported-by: Maxim Samoylov <max7255@yandex-team.ru>
---
 kernel/futex.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index b59532862bc0..8a6d35fa56bc 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -508,10 +508,21 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, enum futex_a
 
 	/*
 	 * The futex address must be "naturally" aligned.
+	 * Also send signal to break busy-loop if user-space ignore error.
+	 * EFAULT case should trigger SIGSEGV at access from user-space.
 	 */
 	key->both.offset = address % PAGE_SIZE;
-	if (unlikely((address % sizeof(u32)) != 0))
+	if (unlikely((address % sizeof(u32)) != 0)) {
+		struct kernel_siginfo info;
+
+		clear_siginfo(&info);
+		info.si_signo = SIGBUS;
+		info.si_code  = BUS_ADRALN;
+		info.si_addr  = uaddr;
+		force_sig_info(&info);
+
 		return -EINVAL;
+	}
 	address -= key->both.offset;
 
 	if (unlikely(!access_ok(uaddr, sizeof(u32))))