linux-kernel - [PATCH v2 3/3] futex, x86/mce: Avoid double machine checks

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Date:   Mon, 11 Jan 2021 13:44:52 -0800
From:   Tony Luck <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-mm@...ck.org
Subject: [PATCH v2 3/3] futex, x86/mce: Avoid double machine checks

futex_wait_setup() first tries to read the user value with page faults
disabled (because it holds a lock, and so cannot sleep). If that read
fails it drops the lock and tries again.

But there are now two reasons why the user space read can fail. Either:
1) legacy case of a page fault, in which case it is reasonable to retry
2) machine check on the user address, bad idea to re-read

Check for the ENXIO return code from the first get_user() call and
immediately return an error without re-reading the futex.

Signed-off-by: Tony Luck <tony.luck@...el.com>
---
 kernel/futex.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index c47d1015d759..b11166712a9f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -723,7 +723,7 @@ static int get_futex_value_locked(u32 *dest, u32 __user *from)
 	ret = __get_user(*dest, from);
 	pagefault_enable();
 
-	return ret ? -EFAULT : 0;
+	return (ret == -ENXIO) ? ret : ret ? -EFAULT : 0;
 }
 
 
@@ -2658,6 +2658,9 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
 	if (ret) {
 		queue_unlock(*hb);
 
+		if (ret == -ENXIO)
+			return ret;
+
 		ret = get_user(uval, uaddr);
 		if (ret)
 			return ret;
-- 
2.21.1